BOTANY

FOR

AGRICULTURAL STUDENTS

BY

JOHN N. MARTIN

Professor of Botany at the Iowa State College of Agriculture and Mechanic Arts

FIRST EDITION

NEW YORK

JOHN WILEY & SONS, INC.

LONDON: CHAPMAN & HALL, LIMITED 1919

COPYRIGHT, 1919,

BY JOHN N. MARTIN

Stanbopc jprcss

F H. GILSON COMPANY BOSTON, U.S.A.

PREFACE

Although students vary widely in their reasons for studying Botany, the fundamental facts or principles of the subject are not thereby altered. One has considerable freedom, however, in the presentation of the subject to adapt the subject matter to special aims of different classes of students, and especially is this true in courses for agricultural students, since much of the work in Agriculture is based upon the principles of Botany. In the choice of material to illustrate principles and in the presen- tation of the applications of principles, there is special oppor- tunity to relate courses in Botany to courses in Agriculture.

In any elementary course in Botany, regardless of the kind of education the student desires to obtain, the primary aim should be to give the student a notion of the fundamental principles of Botany. This aim should be the guiding one in both recitation and laboratory, determining the trend of discussions in recita- tion, and the nature of the material and procedure in the lab- oratory. The primary aim should be accompanied by a secondary aim to relate the subject to the student's major line of work. When the relation of the subject to major lines of work is obvious, the student is more likely to appreciate the subject and is thereby put in a favorable mood to study the subject. Even for students who take Botany merely as a part of a general education, it in no way detracts from the course or makes botanical training less efficient to present the practical aspects of the subject.

This book is intended for elementary courses in Botany in colleges and universities. In its preparation the aim has been to present the fundamental principles of Botany with emphasis upon the practical application of these principles. The subject matter is presented in two parts, part I being devoted to the study of the structures and functions chiefly of Flowering Plants, and Part II, to the study of the kinds of plants, relationships, Evolution, Heredity, and Plant Breeding.

In the preparation of the book, I had the following objects in view: (1) to present the structures and functions of Flowering

iii

iv PREFACE

Plants and relate them to such agricultural subjects as Farm Crops, Forestry, and Horticulture, and to the more advanced courses in Botany; (2) to present the kinds of plants with emphasis upon their evolutionary relationships and their economic im- portance; and (3) to present Evolution, Heredity, and Plant Breeding as related to the improvement of plants.

The topics are arranged in the book in the order in which I usually present them. The presentation of the reproductive structures and processes of Flowering Plants, followed by that of the vegetative organs, has fitted in at Iowa State College with the time of year at which the agricultural students begin the study of Botany and also with the courses in Agriculture. In other schools where conditions are different, other arrangements of the topics are more suitable. In recogni- tion of this fact, most of the chapters have been written so as to be separately understandable, the aim being to make the book adaptable to any arrangement of topics that the teacher may prefer.

In the discussion of a subject the presentation of the general features precedes that of the particular features, and the latter are presented in most cases by the study of type plants chosen on account of their familiarity and economic importance.

The book is intended for an entire year's work in Botany and to be accompanied by laboratory work. Where less time is de- voted to the subject, the organization of the chapters so as to be separately understandable permits a selection of topics according to the requirements of the course.

The reproductive structures and processes in Flowering Plants (Chapters III and IV) are dwelt upon more than is necessary for students who have had a good course in Botany in a high school. A large percentage of the students in my elementary classes have had no Botany and have difficulty in understanding sexual reproduction in Flowering Plants. In an effort to thor- oughly acquaint the student with this subject, I have dwelt at considerable length upon those phases of the subject that are in my experience difficult for the student to understand. In case students are familiar with this subject, parts of Chapters III and IV can be omitted or read hastily in review.

Usually there are some students in the class that are especially interested in certain topics and desire a more complete discussion

PREFACE V

of the topics than the text affords. In recognition of this fact, I have added, chiefly as footnotes, many references. Most of the references are bulletins on the special topics, and in addition to giving further information on the special topics, these references introduce the student to that vast source of information contained in the bulletins published by the U. S. Department and the ex- periment stations of the different states.

Many of the illustrations have been taken from the publica- tions of various authors whose names or the names of their pub- lications appear in connection with the illustrations. To these authors I am much indebted. Most of the original illustrations have been made by Mrs. Edith Martin, who has also given me valuable assistance in other ways in the preparation of the book. Also much credit is due Mr. H. S. Doty and Mr. L. E. Yocum, my assistants, who have given me valuable suggestions. To Dr. L. H. Pammel, who read some of the topics on Fungi and offered valuable suggestions, I am also much indebted.

The book no doubt has many faults, but I hope it has some particular value and that the criticisms which teachers offer will make me a more efficient teacher.

J. N. MARTIN. AMES, IOWA Oct. 7, 1918

CONTENTS

INTRODUCTION

CHAPTER PAGB

I. THE NATURE AND SUBDIVISIONS OF BOTANY 1

II. A GENERAL VIEW OF PLANTS 5

PART I PLANTS (CHIEFLY SEED PLANTS), AS TO STRUCTURES AND FUNCTIONS

III. FLOWERS 9

General characteristics and structure of flowers 9

Some particular forms of flowers 16

Arrangement of flowers or inflorescence 26

IV. PISTILS AND STAMENS 33

Structure and function of pistils and stamens 33

Pollination 46

V. SEEDS AND FRUITS 55

Nature and structure of seeds 55

Resting period, vitality, and longevity of seeds 67

Purity and analysis of seeds 74

Nature and types of fruits of Flowering Plants 77

Dissemination of seeds and fruits 82

VI. GERMINATION OF SEEDS; SEEDLINGS 89

Nature of germination and factors upon which it depends . 89

Germinative processes 93

Testing the germinative capacity of seeds 98

Seedlings 102

VII. CELLS AND TISSUES 112

Structure and function of cells ...» 112

Respiration 121

Cell multiplication 123

General view of tissues 126

vii

yiii CONTENTS

CHAPTER PAGE

VIII. ROOTS .'•'.-• 135

General features of roots 135

Root structure 143

Factors influencing the direction of growth in roots .... 150

The soil as the home of roots 152

Water, air, and parasitic roots * 162

Propagation by roots 163

IX. STEMS 166

Characteristic features and types of stems 166

General structure of stems 166

Structure of monocotyledonous stems 187

Structure of herbaceous dicotyledonous stems 192

Structure of woody stems 197

X. BUDS: GROWTH OF STEMS; PRUNING; PROPAGATION BY STEMS 204

Buds 204

Growth of stems 213

Pruning 221

Propagation by means of stems 225

XI. LEAVES 233

Characteristic features of leaves 233

Primary and secondary leaves 234

General structure of leaves 242

Cellular structure of leaves 246

The manufacture of food by leaves 252

Factors influencing photosynthesis . . 257

Transpiration from plants 260

Respiration 269

Special forms of leaves 270

Uses of the photosynthetic food 273

PART n

PLANTS AS TO KINDS, RELATIONSHIPS, EVOLUTION, AND HEREDITY

XII. INTRODUCTION 289

XIII. THALLOPHYTES 296

Algae (Thallophytes with a food-making pigment) .... 296

General characteristics 296

Blue-green Algae (Cyanophyceae) 297

Green Algae (Chlorophyceae) 301

Brown Algae (Phaeophyceae) 318

Red Algae (Rhodophyceae) 324

Some Alga-like Thallophytes not definitely classified . . 329

CONTENTS IX

CHAPTEB PAGE

XIV. THALLOPHYTES (continued) 336

Myxomycetes and Bacteria (Thallophytes lacking food- making pigments) 336

Myxomycetes (Slime Molds) 336

Bacteria 341

XV. THALLOPHYTES (concluded) 351

Fungi (Thallophytes lacking food-making pigments) .... 351

Phycomycetes (Alga-like Fungi) 353

Ascomycetes (Sac Fungi) and Lichens 363

Basidiomycetes 382

Fungi Imperfecti (Imperfect Fungi) 404

XVI. BRYOPHYTES (Moss PLANTS) 405

Liverworts and Mosses 405

Liverworts 406

Mosses 417

XVII. PTERIDOPHYTES (FERN PLANTS) 425

Filicales 426

Equisetales (Horsetails) 435

Lycopodiales (Club Mosses) 438

XVIII. Spermatophytes (Seed Plants) 445

Gymnosperms (Seeds not enclosed) 445

Cycads (Cycadaies) 446

Pines (Pinaceae) 451

XIX. Spermatophytes (continued) 459

Angiosperms (seeds enclosed) 459

XX. CLASSIFICATION OF ANGIOSPERMS AND SOME OF THEIR FAMI- LIES OF MOST ECONOMIC IMPORTANCE 471

Dicotyledons (Apetalae) . 473

Dicotyledons (Polypetalae) 481

Dicotyledons (Sympetalae) 489

Monocotyledons 495

XXI. Ecological classification of plants 500

Nature of Ecology 500

Ecological factors 501

Ecological societies 504

Plant succession 510

XXII. EVOLUTION 513

Meaning and Theories of Evolution 513

Experimental Evolution 524

XXIII. Heredity. 535

General features of Heredity 535

Experimental study of Heredity 537

X CONTENTS

CHAPTER PAGE

XXIV. PLANT BREEDING 557

Selection 557

Mass culture 558

Pedigree culture 560

Selection of Mutants 561

Hybridization 561

Crossing and vigor of offspring 564

BOTANY FOE AGKICULTUKAL STUDENTS INTRODUCTION

Botany for Agricultural Students

CHAPTER I THE NATURE OF BOTANY

Botany is a branch of Biology which includes all of the sciences that deal with living things. Zoology, Bacteriology, Human Anatomy and Physiology are some other biological sciences that are familiar and closely related to Botany.

The word botany comes from a Greek word, bosko, meaning, " I eat.' ' Botany was originally the science of things good to eat, and in its naming the fact was recognized that plants are the source of our food. Of course at the present time Botany studies all kinds of plants which include besides the many useful for food, many useful as medicine, and many that are poisonous. Botany is commonly defined as that science which treats of plants. This definition is not entirely satisfactory because it does not separate Botany from such agricultural subjects as Horticulture, Forestry, and Farm Crops which also treat of plants.

Between Botany and those agricultural subjects which study plants, there is no sharp division line. Much of the work in these agricultural subjects is based upon the principles of Botany. Such features as plant structures, plant functions, and relation of functions to sunlight, air, soil, etc., which are studied in Botany, are features of consideration in Horticulture, Forestry, and Farm Crops. Although Botany and these agricultural subjects study many plant features in common, the latter subjects differ from Botany in studying only special groups of plants, and in limiting the study to the practical and economic phases of plants.

A plant may be studied in a number of different ways. It may be considered in reference to structure, functions, and in relation to other plants. Botany is divided into a number of subjects which consider different phases of plant life.

1

2 THE NATURE OF BOTANY

MORPHOLOGY considers the form and structure of plants. It considers the forms of plant bodies and the organs and tissues which compose them. Morphology studies the structure of roots, stem, leaves, buds, and flowers, and establishes the rela- tionships of organs. Morphology not only considers the more complex plants but also the simpler ones, and traces the develop- ment of plant structures through the different plant groups. The phase of Morphology in which the development of the more complex plants from the simpler ones is studied, is called Plant Evolution. When Morphology is concerned with the micro- scopical study of the finer structures of plants, then it is called Anatomy, and if the study is mainly concerned with the structure of the cell, then it is called Cytology. Anatomy and Cytology are often spoken of as Histology. Another phase of Morphology is Embryology which, as the term suggests, is the study of the embryo, or the study of the plant during its formation in the seed.

PLANT PHYSIOLOGY studies the functions of plant structures and the relation of these functions to light, temperature, air, soil, etc. It treats of how the plant lives, respires, feeds, grows, and re- produces. In the study of Plant Physiology we learn how plant food is made and transported, and how plants grow. As a basis for the study of Plant Physiology, one must have a knowledge of the Morphology of plants and also a knowledge of Chemistry and Physics.

PLANT PATHOLOGY treats of plant diseases. In this subject one learns the disease producing plants and how they affect the plant diseased. In the study of Plant Pathology, in order to know how the diseased plant is injured, one must know the nature and function of the tissues attacked. This means that one should know Morphology and Plant Physiology. Furthermore, in order to know how the disease producing form attacks other plants and propagates itself, one needs to know its Morphology and Physi- ology.

PLANT ECOLOGY considers plants in relation to the conditions under which they live. Some plants can live on a dry hill top, while others can live only in moist, shady places. Some can live in colder regions than others. Some plants, like many of the weeds, can thrive when crowded among other plants, while some like the Corn plant can not. Marshes, bogs, forests, sandbars, etc., all have their characteristic plants. One set of plants often

SUBJECTS TREATED IN THIS BOOK 3

prepares the way for others. On exposed rocks only very small plants are able to grow at first, but due to their presence soil accumulates and larger plants are able to follow. Such problems as the above are studied in Ecology. Ecology studies plants in relation to the effects of soil, climate, and friendly, or hostile animals and plants. It also studies the effect of the different conditions upon the form and structure of plants.

PLANT GEOGRAPHY is much like Ecology and treats of the dis- tribution of the different kinds of plants over the earth's surface.

TAXONOMY, or SYSTEMATIC BOTANY, treats of the classification of plants. As a result of this kind of study, plants have been arranged in groups, such as Algae, Bacteria, Fungi, Mosses, Ferns, and Seed Plants. These large groups are further sub- divided into smaller groups. Keys have been arranged by which plants unknown to the student may be identified. Through the study of Systematic Botany one can learn the names and some of the characteristics of the different kinds of Grasses, weeds, shrubs, and trees that grow on the farm or in any other region.

ECONOMIC BOTANY treats of the uses of plants to man.

PALEOBOTANY is concerned with the history of plants as shown by their preserved forms, known as fossils, which occur in the different layers of rock composing the earth's crust. Paleobotany is studied in connection with Geology. In the study of this subject much has been learned about the plants which lived millions of years ago, and this knowledge is very useful in under- standing the evolution of the plants which now exist.

Subjects treated in this Book. — To become a master in any one of the above subjects would require years of one's time. A study of any of the special subjects of Botany requires a general knowledge of the anatomy and the functions of plant struc- tures This means that one must have a general course in Botany before making a special study of Morphology, Plant Physiology, or any of the special botanical subjects. • One purpose of this book is to give a general knowledge of cultivated plants, of plants not cultivated but like the Rusts and Smuts related to Agriculture, and of those plants which one must know in order to understand the evolution of plants. Another purpose is to give such a general knowledge of plant anatomy and the functions of plant structures, that one will have the necessary knowledge for the study of such agricultural subjects

4 THE NATURE OF BOTANY

as Horticulture, Forestry, and Farm Crops, and also a basis for the study of the special botanical subjects. These special subjects of Botany are not only very important to one who makes a special study of Botany, but some phases of Morphology, Plant Physiology, Plant Pathology, Systematic Botany, and Ecology are important studies for agricultural students in certain agricultural courses. Part I, this book, deals mainly with the parts of plants as to structure and function and, therefore, emphasizes the Morphology and Physiology of plants. But structure and function as well as other aspects of plants, accom- pany and explain each other and can not well be separated in an elementary study of Botany. So the different phases of the plant are studied as they occur in relation to each other and without any designation as to whether or not the fact belongs to Morphology, Physiology, or any other special phase of Botany. Part II is devoted chiefly to a study of plants as to kinds, rela- tionships, evolution, and heredity.

CHAPTER II A GENERAL VIEW OF PLANTS

Abundance and Distribution of Plants. — Plants are so abun- dant and generally distributed that there are very few regions that do not have plants. Plants occur in the water and in the soil as well as on the surface of the earth. Some plants live in the bodies of animals. Some are able to live where the tem- perature is intensely cold, while others can live in hot springs where the temperature is not far from the boiling point. Even on rocks that look quite bare, a close examination will show that some plant forms are present. Only in exceptional places, such as volcanic regions, some hot springs, and regions of salt deposits, are plants generally absent.

The abundance or scarcity of plants in a given region depends upon how well the conditions of the region meet the requirements for plant growth. If the soil is dry, as in desert regions, the average number of plants per area is usually quite small, while in regions where there is sufficient moisture and sufficient mineral substances, more than 100,000 plants may occur on an area no larger than an average garden. However, the number of plants which can occur on a given area, is often very different from the number that can do well on this same area. Many more grain plants can be grown per acre than are grown, but agriculturists have learned that only a limited number of plants per acre can do well. Among plants, as among animals, there is competition. Plants must compete with each other for moisture, mineral substances, and sunlight, and when the competition is too great, as occurs when plants are too much crowded, some or all of the plants suffer and fail to produce good yields. By controlling the amount of seed sown and by properly distributing the seed, the farmer is able to raise the greatest number of plants per acre with the least loss from competition among the plants.

Diversity of Plant Forms. — Plants are not only the smallest, but also the largest of living organisms. Many plants are so

5

6 A GENERAL VIEW OF PLANTS

small that they can be seen on y with a microscope. Ranging from these very small plants to the largest trees, plants of all sizes and complexity occur about us. The different plant forms differ very much in structure, methods of getting food, and methods of reproduction. The plants which concern us most are those which have flowers. They are known as the Flowering Plants. Most of the cultivated plants and nearly all weeds belong to this group. They are the plants which furnish nearly all of our food and fibers and much of our lumber. Part I of this book is devoted to the study of the Flowering Plants.

Although the Flowering Plants concern us most, it must not be concluded that the simpler plants are of no importance. The simpler plants, even the microscopic forms, not only help and hinder in the cultivation of the Flowering Plants, but affect us in other ways and must receive consideration. Much of Part II is devoted to the study of them.

Parts of a Plant. — In plants, as in animals, there is a living body consisting of parts each of which has a special work to perform. The various parts of a plant having their own special work are called organs, and the special work of an organ is its function. Plants, like animals, being composed of organs, are called organisms. In the Flowering Plants, the plant body con- sists of roots, stem, leaves, buds, flowers, seeds, and fruit. All of these structures are not present at all times, but unless a Flowering Plant develops all of these organs during its life, its development is considered incomplete. Through the special functions of its organs, the plant is able to exist and reproduce itself. The roots hold the plant to the soil and furnish water a'nd salts; the stem supports the leaves, flowers, and fruit in the air and sunlight; the leaves make food; the buds produce new leaves and flowers; and the flowers, seed, and fruit have to do with the production of new plants. But each organ is also composed of parts and to understand an organ one must understand its special groups of cells, known as tissues, of which the organ is composed.

Life Cycle of Flowering Plants. — A characteristic of living organisms is their ability to use substances as food, grow, and develop. Living organisms are also much influenced by their surroundings. Plants are much influenced by the nature of the soil, air, sunlight, and plants which grow about them.

To understand a plant one needs to study it in its various

LIFE CYCLE OF FLOWERING PLANTS 7

stages of development. The tiny Corn plant, called embryo or germ, which we find in the Corn kernel, does not look much like the plant that bears tassel and ears. From the embryo to the flower and seed stage, many things take place. The series of events which take place in the development of the embryo to a mature plant constitutes the life cycle of a plant. Starting from

FIG. 1. — Life cycle as illustrated by the Corn plant, a, mature kernel; 6, germination; next, seedling; d, mature plant composed of roots, stems, leaves, and flowers, all of which are composed of tissues having special func- tions to perform; e, the two kinds of flowers with pollination indicated; /, fertilization indicated by the two globular bodies, sperm and egg, on the inside of the ovary or portion that develops into the kernel. After ferti- lization the ovary develops into another kernel and thus the life cycle is completed.

the seed, this series of events consists of germination, develop- ment of seedling with its different organs and tissues, develop- ment of root, stem bud, and leaf structures of the more mature plant, development of flowers, pollination and fertilization, and development of other kernels. The life cycle of any Flowering Plant is similar to that of the Corn. Thus it is seen that the life

8 A GENERAL VIEW OF PLANTS

cycle of a plant returns us to the place of starting. The series of events may be represented as shown in Figure 1, and in tracing them one can begin at any point. The yield of the plant at maturity depends upon how well it has done at the different stages in its life cycle. The purpose of cultivation is to help the plant to do well at all stages, and it is for this reason that we look after the fertility of the soil, select seed, prepare a seed bed, sow or plant a certain amount of seed and in a certain way, prevent the growth of weeds, etc. But often methods of cultivation must take into account the structure and function of plant organs as they occur at the different stages in the life cycle of the plant, and unless the peculiar features of the plant are understood, the methods employed in cultivation may not be adapted to secure the best results.

PAET I

PLANTS (CHIEFLY SEED PLANTS) AS TO STRUC- TURES AND FUNCTIONS

CHAPTER III

FLOWERS General Characteristics and Structure of Flowers

On account of their colors and odors, flowers very much excel other plant organs in attracting attention. Everybody is in- terested in flowers on account of their aesthetic charm, if for no other reason. The attractive colors and pleasant odors common to flowers not only interest the scientist but also appeal to the aesthetic sense of people in general. In fact many people would define the flower as the showy part of the plant. However showiness is not an essential feature, for there are many flowers which have no attractive colors or odors and yet they are just as genuine in function as are showy flowers. Most forest and shade trees, the Grasses, and many weeds do not have showy flowers. The flowers of such plants as the Oaks, Elms, Maples, and Pines lack showy parts and are so inconspicuous that most people have not noticed them, yet these flowers are just as genuine in function as those of a Lily or Rose.

On account of their showiness and importance in reproduction, flowers were first to receive careful study; and in the early history of Botany, flowers were about the only plant structures that received much attention. At the present time there are some people who have the erroneous notion that the study of Botany and flowers are still almost identical despite the fact that the study of flowers is now of no more importance than many other phases of plant life, as is well shown by the large amount of space devoted by our present botanical texts to the study of roots, stems, leaves, and other phases of plants.

In size, flowers may be almost microscopical as in some of the small floating water plants, such as the Duckweeds, or they may be of huge dimensions as some tropical flowers which are two or more feet across. Even in the ordinary greenhouse, some flowers are so small that they are not conspicuous except in large clusters,

9

10 FLOWERS

while those of Carnations and Roses are conspicuous when single. In Chrysanthemums, Daisies, and Sunflowers the individual flowers, although small, form a cluster so compact that it is often erroneously considered a single flower.

As to color, which is the character most closely related to securing pollination by insects, flowers are exceedingly various. Some, especially those that depend upon the wind for pollination, are green like leaves. Some are white, while among others nearly every color imaginable can be found. It is claimed that by means of colors flowers solicit the visitation of insects, which are im- portant agents in pollination.

The odors of flowers, usually pleasant, but sometimes repul- sive to us, as in case of the Carrion-flower and Skunk Cabbage, probably serve in attracting insects. Further- more, pleasant odors add

P il^iil Mj^Z''' to the value of plants for

ornamental purposes.

Flowers present various •p forms. When well open,

FIG. 2. - Basswood flower with portions some are wheel-shaped, removed from one side so that the interior some funnel-shaped, some of the flower may be seen, a, calyx com- tubular, while others de- posed of leaf-like portions or sepals; o, part from these forms with corolla composed of leaf-like portions called varioug ; lariti ag in

petals; s, stamens; p, pistil; r, receptacle. .

Much enlarged. the Sweet Pea> where the

flower resembles a butter- fly in shape, or in the Orchids where parts of the flower may be so shaped as to resemble a slipper, as the Orchid known as the Lady's-slipper illustrates. The shape of the flower in many cases favors the visitation of only special insects, and, therefore, is closely related to the problem of pollination.

To discover the essential features of a flower, it becomes necessary to determine the function of the flower, and become acquainted with its parts and the use of each part in relation to the work of the flower.

Function of the Flower. — The flower is the plant's principal organ of reproduction, being devoted to the production of seed which is the plant's principal device for producing new plants.

PARTS OF THE FLOWER 11

Functionally, the flower may be defined as the organ which has to do with seed production. Flowers which have been so modi- fied through cultivation that they no longer produce seed are not true flowers. However, the true function of the flower is often not the important feature to the plant grower. Many flowers are cultivated entirely for their aesthetic charm. In case of fruit trees, Tomatoes, and many other plants, the structure developing from the flower and known as the fruit is more important to the plant grower than the seed. However, when plants are grown for seed or fruit, the amount of seed or fruit harvested depends very much

upon the number of flowers pro- / ^ / llf^V^\ duced. For example, the gar- ^^""^^ II * ^^. \. dener does not expect to gather many Beans or Peas if the FIG. 3. -Apetalous flower of Buck- , , P a wheat, c, calyx; s, stamens: p. pistil;

vines produce only a few flowers. r> receptac'le. Much enlarged. After Likewise, good crops of Clover Marchand. and Alfalfa seed depend upon a

good crop of flowers; and not much fruit is expected when the flowers in the orchard are few. It is in connection with the function of reproduction, that flowers have developed the various colors, forms, and odors which assist in bringing about fertiliza- tion, the central feature of sexual reproduction to which the flower is devoted, and the process upon which the development of seed usually depends.

Despite the multitudinous forms and colors which flowers present; there is much unity and simplicity in structure, all parts being organized to assist in performing the function of seed production.

Parts of the Flower. — The parts of a flower are of two general kinds; those which are directly concerned in the production of seed; and those which act as protective and attractive organs. The former are known as the essential organs, and consist of stamens and pistils. The latter are known as floral envelopes or perianth, and usually consist of two sets of organs, one called calyx and the other, corolla. In Figure 2, the calyx is the lowest whorl and consists of green leaf-like portions called sepals. The

12

FLOWERS

second whorl is the corolla and each separate portion is a petal. The pistil occupies the central position and is surrounded by the whorl of stamens. The end of the flower stem to which these

FIG.' 4. — A flower of Tobacco. c,the FIG. 5. — Flower of Red Clover,

funnel-shaped corolla made up of united c, corolla; 6, cup-like calyx. Much

petals; 6, calyx. The sepals are also enlarged. After Hayden. united below. Reduced.

floral parts are attached is called torus or receptacle. The receptacle may be flat, conical, or cup-shaped, and often forms

FIG. 6. — The two unisexual flowers of the Pumpkin with a portion of the bell-shaped corollas torn away to show the interior of the flowers.

A, staminate flower; s, stamens fitting together, forming a column. B, pistillate flower. Less than half natural size.

an important part of the fruit. The corolla is usually bright colored, and, therefore, the conspicuous part of the flower. It is also the fleeting part of the flower, usually lasting only a few days.

UNISEXUAL FLOWERS

13

s-

P-

FIG. 7. — Section

Flowers having the four sets of organs, as shown in Figure 2, are called complete flowers to distinguish them from incomplete flowers, that is, flowers in which some of the organs are lacking. The organs are gener- ally arranged in a circular fashion around the receptacle, and are characterized as be- ing in cycles or whorls. In some flowers a part or all of the perianth is lacking. In the Buckwheat, as shown in Figure 3, only one whorl surrounds the stamens and pistil, and it is evident that this flower does not have both calyx and corolla. In such cases, the petals are considered missing and the flower is said to be apetalous (" without

petals")- Often instead of being composed

c ,• -i , i / i - i \ through a flower of the

of entirely separate petals (polypetalous), Peac£ There ig but

the corolla is a tube or funnel-shaped struc- One pistil (p), but many ture, which appears to be composed of united stamens (s) . Much en- petals (gamopetalous) , separate only at the lar£ed- top. (Fig. 4-) The flowers of the Tobacco Plant, .Pumpkins,

Squashes, and Water- melons are examples of gamopetalous flowers. In some cases, as in the Tobacco, Clover, and some other plants, the sepals seem to have joined into one structure (gamosepalous), forming a tube- or cup-like calyx. (Fig. 4 and 5.) Flowers also differ in the essential organs contained. FIG. 8. — Section through an Apple flower Unisexual Flowers. — showing the compound pistil composed of five Flowers having both sta- carpels. The five carpels (a) are free above mens and pistils are but joined below, c, corolla; s, stamens; i, known as perfect Or bisex- calyx. Much enlarged. 7 n T

ual flowers. In some

plants, the stamens and pistils occur in different flowers, in which case the flower having stamens only is called a staminate flower,

14

FLOWERS

while the other having pistils only is called a pistillate flower. Such flowers are said to be unisexual. Pumpkins, Cucumbers, Corn, Hemp, Willows, and Poplars are some of the familiar plants which have unisexual flowers. In Figure 6 are shown the uni- sexual flowers of the Pumpkin. In some cases, as in Corn, Cucumbers, and Pumpkins, both staminate and pistillate flowers

are borne on the same plant. Such plants are said to be monce- cious (meaning " of one house- hold "). In other cases, as in Hemp, Willows, and Poplars, the staminate and pistillate flowers are borne on different in- dividuals, that is, one plant has FIG. 9.— Section through the flower OI^Y staminate while another has of Cotton, s, stamens joined into a only pistillate flowers. Such tube which surrounds the pistil; p, plants are said to be dioecious pistil composed of carpels more united (meaning « of two households ") . than those of the Apple. Smaller p.. ... , 0, .

i • * *r ™ MI Pistils and Stamens. — As

than natural size. After Baillon.

everyone knows, the pistils are

the organs in which fertilization occurs and seed is produced, while the stamens furnish the pollen, which is essential for fertilization. Flowers usually have more stamens than pistils, but the number

FIG. 10. — A flower of a Legume with petals removed to show the dia-r delphous stamens, a, free stamen; 6, tube formed by the joining of the other stamens.

of each varies much in the flowers of different plants. Some flowers, as those of the Strawberry, have numerous stamens and pistils, while in some flowers, as in the Peach or Plum, there is only one pistil, but many stamens. (Fig. 7.) The Apple flower, which has many stamens, really has five pistils, but the lower parts of the pistils are joined, leaving only the upper parts free,

PISTILS AND STAMENS 15

A pistil like that of the Apple is called a compound pistil, and the pistil-like structures which compose it, instead of being called pistils, are called carpels. Thus in Figure 8, each of the branches in the upper region of the pistil is the upper portion of a carpel. If the enlarged bases of these were separated, then each carpel would resemble the pistil of the Cherry or Plum flower. Pistils like those of the Cherry and Plum consist of only one carpel and are, therefore, called simple pistils. In flowers having but one carpel,, pistil and carpel mean the same thing. The flower of the Cotton Plant, shown in Figure 9, has a compound pistil in which the carpels are more united than in the Apple.

In most flowers the stamens are separate from one another (polyadelphous), but in some groups of plants they are more or

A

FIG. 11. — A, hypogynous flower of Pink; B, perigynous flower of Cherry; C, epigynous flower of Wild Carrot. Modified from Warming.

less united (monadelphous) . In Cotton and other plants of this group, the stamens are joined in such a way as to form a tube around the pistil. (Fig. 9.) In Clover, Alfalfa, and some other plants of this family, the ten stamens form two groups (diadel- phous), nine being joined and one remaining free.

The relative positions of the different parts of the flower show considerable variation. In some flowers, as those of the Dande- lion or Sunflower illustrate, the calyx, corolla, and stamens arise from the top of the ovary. (Fig. 2 4.) Such flowers are epigy- nous, i.e., the floral structures are on the gynous the word " gynous " referring to the ovary, which in this case is described as inferior. In the Basswood flower, calyx, corolla, and stamens are attached to the receptacle at the base of the ovary, which is

16

FLOWERS

described as superior. Such flowers are hypogynous. In some flowers, as in the Peach shown in Figure 7, the calyx, corolla, and stamens are at- tached to the rim of a cup- like structure surrounding the ovary. In this case the flower is perigynous, and the ovary is described as half inferior. To which of the above classes does the Apple flower belong? In Figure 11 the three positions of the perianth and stamens in reference to the ovary are shown for comparison.

Some Particular Forms of Flowers

That there are numerous differences among flowers is shown by the fact that largely upon differences pertaining to flowers, the Flowering Plants have been divided into many classes, such as orders, which in turn are subdivided into fami- lies, then into genera, and finally into species of which there are more than 100,000. The differences are mainly struc- tural, and between flowers of FIG 12. -Corn _ plant t, tassel d;fferent famUies ^ are <rften

consisting of staminate flowers; e,

ears on which the pistillate flowers qulte prominent. For example, are found. when such flowers as those of the

Grass, Bean, Sunflower, and

Orchid family are compared, that there are peculiar differences in the character of flowers is obvious.

Grass Flowers. — One of the characteristic features of the Grass flowers is, that there are no showy organs. Grass flowers

CORN FLOWERS

17

are usually green like leaves, and their stamens and pistils are enclosed and protected by small leaf-like bodies called bracts, which take the place of a calyx and corolla. Although quite inconspicuous, yet in being characteristic of such Grasses as Corn, Wheat, Oats, Barley, Rye, Rice, and Timothy, Grass flowers are so im- portant that they deserve some special attention.

Corn Flowers. — As already stated (page 14) Corn flowers are unisexual. The stami- nate flowers are produced in the tassel, while the pistillate flowers occur on the ear. (Fig. 12.)

The staminate flowers bear three stamens and occur in groups of twos, called spikelets. The branches of the tassel upon which the spikelets are crowded are known as spikes. In Figure 18 is shown a spike or branch of the Corn tassel so drawn as to show the spikelets.

The two flowers of each spikelet are in such close contact, that in order to identify each Corn tassel, sp, flower, the bracts must be spread apart as spikelets. Only three shown in Figure 14- In the older flower, the of tne spikelets are stamens have elongated and pushed out of the bracts. The boat-shaped bracts are so fitted together as to make a good enclosure for the stamens during their development. The two outer bracts, situated on opposite sides of the spikelet and facing each other, so as to close together and enclose the flowers, are known as glumes. Between each glume and set of stamens is the bract called lemma. The bract on the opposite side of the stamens, with its concave side turned toward that of the lemma, is known as the palea. The palea and lemma, when closed against each other, enclose the stamens. The small bodies at the base of the stamens are called lodicules, and may, by their swelling, spread the bracts apart, thus helping the stamens to escape from their enclosure. The structure of the flower will be more easily understood by a study of Figure 14. The glume is not considered a part of the flower. The two glumes form a covering for the spikelet.

FIG. 13. — A branch spike from the

Sligh%

18

FLOWERS

Other names are often applied to the glume and lemma. In courses in Agriculture, the glume is often called outer or empty glume and the lemma, the flowering glume.

The pistillate flowers are arranged on a cob and enclosed by husks, so that only the outer ends or silks of the pistils are

FIG. 14. — A spikelet from the Corn tassel. Much enlarged to show the two staminate flowers

The flowers are numbered (1} and (#), No. 1 being more mature, e, glumes; /, lemma; p, palea; s, stamens; I, lodicules.

exposed. When the husks are removed, the flowers are seen arranged on the cob just as the kernels are in the mature ear, for each kernel develops from a flower. Explain what is shown in Figure 15. The pistillate flowers occur in groups of two's or spikelets, but only one flower of the » spikelet completes its development. The flower which remains rudimentary develops no silk and remains so inconspicuous that one needs a magnifier to see it. Since it has no pistil, its presence is known only by its bracts. In Figure 16, point out the rudimentary flower and the one that develops.

CORN FLOWERS

19

-t

FIG. 15. — Lengthwise section through the end of a young ear of Corn, showing the spikelets containing the pistillate flowers. h, husk; s, silks of the pistils; 6, enlarged bases of the pistils en- closed by bracts; c, cob. Slightly enlarged.

FIG. 16. — A spikelet from a young ear of Corn to show the two pistil- late flowers. I, the bracts of the flower that develops no pistil. The other bracts belong to the flower having the pistil, r, ovary which becomes the kernel; t, style of the silk; s, the branched stigma; e, glumes; /, lemmas; pa, paleas. The lodicules are very small and are not shown. Very much enlarged.

20

FLOWERS

A study of Figure 16 shows that the base of the pistil is sur- rounded by bracts, corresponding to those surrounding the stamens in the staminate flowers. The bracts of the pistillate flowers are small, membranous, and form the chaff of the cob.

Oat Flower. — A head of Oats, as shown in Figure 17, is much branched and the spike- lets occur at the ends of the branches. Each spikelet con- sists of two or more flowers, which are well enclosed by the two glumes. When the glumes are spread apart as shown in Figure 18, it is seen that the flowers are attached, one above another, to a small slender axis. This axis is known as the ra- chilla. Rachilla means small rachis." Rachis is the name applied to the main axis of the Oat head from which the branches arise. The small branches bearing the spike- lets at their ends are called pedicels. Thus branches arise from the rachis and end in the rachilla to which the flowers of the spikelets are attached.

The spikelet shown in Figure 18 contains three flowers, but the upper one is rudimentary and, therefore, produces no grain. There is one very important difference between the flowers of Oats and those of Corn. In Corn the pistils and stamens occur in different flowers, but in Oats the stamens and pistils occur to- gether in the same flower. The Oat flower is, therefore, a perfect or bisexual flower. In each Oat flower there is one pistil and three stamens enclosed by the lemma and palea. The lodicules, which are two small scale-like bracts at the base of the pistil and stamens, are not easily seen in the Oat flower. The two glumes of the Oat spikelet are so large that when closed together they ti

FIG. 17. — Head or panicle of the Oat plant, s, spikelets; 6, branches; r, rachis; p, pedicels*. About one-half natural size.

OAT FLOWER

21

-I

FIG. 18. — Spikelet of the Oat head, with the bracts spread apart to show the flowers. There are three flowers, only (1 ) and (2} of which develop and produce kernels, e, glumes or empty glumes; /, lemma or flowering glume; pa, palea; s, stamens; p, pistil; r, rachilla. The parts of flowers (2) and (3) are not indicated. Many times enlarged.

FIG. 19. — Two views of a head of Wheat with some spikelets removed to show the zig-zag rachis. An edge view of the spikelets is shown at the left and a side view at the right, r, rachis; s, spikelets.

22 FLOWERS

almost completely enclose the flowers of the spikelet. In thresh- ing most varieties of Oats, only the glumes are removed, the kernel still remaining enclosed by the lemma and palea, which form the covering known as the hull of the grain. A grain of Oats, therefore consists of the kernel and its hull; and the quality of Oats depends much upon the proportion of hull to kernel. As indicated in Figure 18, the lower flower grows

FIG. 20. — Spikelet of Wheat much enlarged and shown with the bracts spread apart, so that parts of the flower may be seen. The flowers are num- bered and the parts of one flower are labelled, e, outer glumes; /, lemma; pa, palea; p, pistil; s, stamens; I, lodicule; a, awn or beard; r, rachis.

more rapidly than the others and forms the larger kernel to which the smaller one sometimes remains attached after threshing.

Wheat Flowers. — In Wheat the head, usually called spike, consists of many spikelets arranged in two rows along the zig-zag axis of the head. (Fig. 19.) This zig-zag axis is the rachis of the spike. The spikelets are not borne at the ends of branches

FLOWERS OF THE LEGUMES OR BEAN FAMILY

23

..a

as in Oats, but are directly attached to the rachis. This feature distinguishes the spike from the branching head, called panicle, of the Oats. In the varieties of common Wheat, each spikelet contains three or more flowers arranged one above another on the rachilla, and one or more of the upper flowers are rudimentary. Each fully developed flower, just as in Oats, consists of three stamens and a pistil enclosed by the lemma and palea. The lodicules, like those of the Oat flower, are small inconspicuous scales at the base of pistil and stamens. In Wheat, where the spikelets are broad, the spikelet is only partly enclosed by the glumes. In thresh- ing Wheat the kernel is separated from the bracts — the latter being blown away as chaff.

A study of the spikelet shown in Figure 20 will aid the student in un- derstanding the structure of Wheat flo.wers and their arrange- ment in the spike- let.

Flowers of the Legumes or Bean Family. — The

fl o w e r s of the

FIG. 22. — End view of an un- tripped and tripped flower of Red Clover.

b, flower untripped. a, stand- Peas, Clover, Al- ard; w, wings; k, keel, d, flower falfa, and Vetch tripped, in which case the keel and are faminar representatives have a wings are bent down, exposing the h f npollijar features The

• i-i / \ j j. / \ iv/r i- IlUIIlUtJl Ul UcLU.llctI IcoiLliltJo. -L lie

pistil (p) and stamens (s). Much

enlarged. After C. M. King. one most prominent among the

cultivated ones of the family is the

irregularity in the shape of the parts of the perianth, as the flowers of Peas or Red Clover illustrate. The calyx is a shallow five-toothed cup. The corolla is composed of four pieces; the large expanded portion at the back, known as the standard or

-ca

FIG. 21. — Flower of Red Clover, ca, calyx; co, corolla; a, standard; w,

Bean Family of wings; k, keel.

which Beans,

Many

times en- After C. . King.

24

FLOWERS

banner; the two side pieces, known as wings; and the single boat-shaped portion beneath the wings, known as the keel. In the Red Clover flower shown in Figure 21 , these parts are pointed out. The stamens and pistil are entirely enclosed by the keel, and when pressure is exerted on the keel, the stamens and pistil spring out of their enclosure with considerable force. (Fig. 22.)

B <-

FIG. 23. — Flowers of the Yarrow (Achillea millefolium), a Composite. A, a head of flowers sectioned, showing the strap-shaped flowers around the margin and the tubular flowers occupying the central region of the head. B and C are tubular' and strap- shaped flowers more enlarged

FIG. 24. — A, flower from the head of Dandelion, a, strap-shaped corolla; 6, calyx made up of many slender hairs known as pappus; p, base of pistil; s, stamens forming a tube around the upper part of the pistil. B, tubular flower and fruit of Beggar's Tick showing tubular corolla (a) and the calyx (6) consisting of two spiny teeth which persist and aid in scattering the fruit.

This process of releasing the stamens and pistil, known as 11 tripping the flower," is mainly the work of insects and is im- portant, because in some of the Legumes the flowers will produce no seed unless tripped.

Composite Flowers. — There is a large group of plants to which Lettuce, Dandelions, Sunflowers, Beggar's Tick, Thistles,

COMPOSITE FLOWERS

25

and many other plants belong, that have their many small flowers grouped in a compact head as shown at A in Figure 23. This group of plants is called Composites, and includes some of our useful plants as well as some of the most troublesome weeds.

FIG. 25. — A cluster of Lady's-slippers.

Both calyx and corolla are somewhat peculiar. In some cases, as in the Sunflower, the flowers occupying the center of the head have tube-like corollas and are called tubular flowers, while those around the margin have strap-shaped and much more showy corollas, and are called ligulate flowers. See A, B, and C of Figure 23. In some of the Composites, as in the Dandelion, all of the flowers of the head are ligulate, while in some, like the Thistle, all the flowers are tubular. The calyx is often composed

26

FLOWERS

of hair-like structures called pappus, as shown in Figure 24. In some, as the Dandelion illustrates, the pappus remains after the seed is mature, forming a parachute-like arrangement which assists in floating the seed about. In some of the Composites, the calyx consists of a few teeth, which in the Spanish Needles

and Beggar's Tick, become spiny, and thereby assist in seed distribution by catching onto passing objects.

Orchid Flowers. — It is among Orchid flowers, many of which are spectacular, that the most notable irregu'arities occur. Besides the dis- tinguishing feature of having the stamens and pistil joined into one body, known as the column, Orchid flowers often have pronounced varia- tions in the shape and size of petals. In some, as in the Lady's-slipper, one of the petals is developed into a great sac or " slipper," while the others have no extraordinary features. These peculiarities in flower structure, which Turnip (Arisoema triphyllum). are apparently adjustments for insect The flowers shown are pistil- pollination, sometimes so closely con- late and are clustered at the r ,, , ji_i_-.r base of the fleshy axis or form to the shaPe and hablt of cer- spadix which is enclosed in the tain insects that only one or a few large leaf-like bract or spathe. kinds of insects can pollinate a flower. Reduced about one-half. guch highly modified flowers contrast

strikingly with the simple, inconspicuous flowers of such plants as the Jack-in-the-pulpit or Indian Turnip and Skunk Cabbage, in which a perianth is either lacking or inconspicuous and the flowers are crowded on a fleshy spike, known as a spadix, which is enclosed in or attended by a leaf, called spathe. The spathe, by becoming colored, often aids like a corolla in attracting insects. (Figs. 25 and 26.}

Arrangement of Flowers or Inflorescence

The arrangement of flowers on the stem is one of the floral characters much used in the classification of the Flowering Plants. In the arrangement of flowers, a number of things are considered,

FIG. 26. —The ous flowers of

uiconspicu- the Indian

ARRANGEMENT OF FLOWERS OR INFLORESCENCE 27

the principal ones being: (1) the position of the flower on the stem, whether terminal or lateral; (2) whether the flowers are single or in clusters; (3) whether the terminal or lateral flowers of a cluster open first; and (4) the character of the cluster in regard to shape and compact- ness, which depend upon the elongation of the stem region bearing the flowers and the length of the individual flower stalks. These features taken singly, to- gether, and along with some minor features form the basis upon which floral arrangements are classified.

Flowers develop from buds and buds are either terminal or •lateral on the stem. So as to position, flowers are either ter- minal or lateral on the flower axis. FlG- 27- — Solitary terminal flower Flowers borne singly are called

of a Lily. After Andrews.

solitary flowers, and solitary flowers may be terminal, as in some

FIG. 28. — A portion of a Squash plant showing the axillary arrangement of flowers. Much reduced.

Lilies of which the Tulip is an example, or lateral, as Squashes illustrate. (Figs. 27 and 28.)

The flower cluster may be regarded as a modification of that lateral arrangement, in which the flowers are scattered on a fully

28

FLOWERS

elongated stem bearing normally developed leaves in the axils of which the flowers occur. Thus, if a Pumpkin or Gourd vine should remain short, the flowers instead of being well separated as they normally are, would be crowded, and, with the reduction of leaves to bracts, a typical flower cluster would result. Most small flowers are produced in clusters. For small flowers polli- nated by insects, there is considerable advantage in the cluster

habit, since the cluster, being much more conspicuous than the individual flowers, serves well as an attractive device.

Flower clusters are divided into two main classes according to their method of development. In the corymbose or indeterminate cluster, growth at the tip and the develop- ment of new flowers just behind continues throughout a considerable period, thus producing a cluster in which the older flowers are left farther and farther behind. As the term indeterminate suggests, such a method of development permits a rather indefinite expansion of the cluster. In the cymose or determi- nate cluster, the oldest flower is formed at the tip, which is thereby closed to further growth, and the new flowers are formed from buds developing lower down. Such a cluster is much limited in its power to expand. The flower clusters of Apples and Pears, known as cymes, illustrate the determinate type of cluster.

The simplest form of the indeterminate cluster is the raceme, an unbranched cluster in which the flowers are borne on short stalks. The racemes of the Shepherd's-purse, Radish, Cabbage, and others of the Mustard family, in which the flower cluster may continue its expansion for a long period, producing new flowers at the tip while pods are maturing at the base, well illustrate the nature of the raceme. (Fig. 29.} The racemes of

FIG. 29. — Raceme of Com- mon Cabbage (Brassica). From Warming.

ARRANGEMENT OF FLOWERS OR INFLORESCENCE 29

the Snap-dragon, Sweet Clover, and Alfalfa are examples of racemes with a short growth period. Racemes may be terminal or lateral, as in case of Sweet Clover.

FIG. 30. — A, spike of Rye. B, panicle of Grass. C, flowers of the Hazel with staminate flowers in catkins and the pistillate flowers borne singly.

FIG. 31. — A, head of Clover. B, close head of Yellow Daisy.

Raceme-like clusters in which the flowers have very snort stalks or none at all are called spikes of which the heads of Wheat and Timothy are familiar examples. A special form of the spike

30 FLOWERS

is the catkin in which the flowers, unisexual in typical cases, usually have scaly bracts instead of a true perianth, and the whole cluster falls after fruiting. Catkins are typical of Poplars, Willows, Hickories, and Birches. When the raceme is so short that the compact mass of flowers form a more or less rounded cluster as in Red Clover, then a head is formed. In the Composites there is the special kind of head which is the most highly organ- ized of all flower clusters. The flowers besides often being differ- entiated into two kinds are so compactly arranged as to form a cluster resembling a single flower and the cluster is surrounded by bracts, which form a structure known as the involucre. (Fig. 31.)

B

FIG. 32. — A, Corymb of one of the Cherries. B, umbel of a species

of Onion.

In contrast to the spike there are those raceme-like clusters in which the flowers have long stalks, as in the typical panicle, where the cluster is loosely branched. When the portion of stem to which the flowers are attached is short and the stalks of all of the flowers are so elongated as to bring all of the flowers to about the same level then a corymb results. A further modification in which the portion of stem to which the flowers are attached is so short that the flower stalks appear to be of the same length and attached in a circle around the stem results in the umbel, the form of cluster characteristic of the Parsley Family, called Umbellif- erce, on account of the character of the flower cluster. Of this family the Parsnips, Carrots, and others are common. The um- bel is also common among the Milkweeds. Umbels may be simple or compound, that is, so branched as to be composed of a number of small umbels. (Fig. 32.}

!* •

ARRANGEMENT OF FLOWERS OR INFLORESCENCE 31

FIG. 33. — A, cyme of the Apple. B, thyrse of the Lilac.

32

FLOWERS

In complex flower clusters combinations of the simpler types of clusters often occur together. Thus, in the thyrse, the complex cluster which is typical of the Lilac and Horse-Chestnut, and, in

a

FIG. 34. — Upper diagrams show types of indeterminate inflorescences, o, raceme; 6, corymb; c, compound corymb; d, umbel; e, spike; /, panicle; g, head.

Lower diagrams show types of determinate inflorescences; h, cyme half developed (scorpioid); i, flat-topped or corymbose cyme; ,;', typical cyme.

the panicle of the Grasses, the characteristics of both racemes and cymes are present. (Fig. 33.)

The diagrams in Figure 34 show the common types of flower arrangements.

CHAPTER IV PISTILS AND STAMENS

Structure and Function of Pistils and Stamens

The pistils and stamens are the organs upon which the pro- duction of seed depends and for this reason are called the essential parts of the flower. The calyx and corolla protect the essential organs and often assist in seed production, but they are not essential.

In unisexual flowers, seeds appear only in the flowers having pistils. The staminate flowers in the Corn tassel produce no kernels, and in dioecious plants, such as Hemp, Willows, and the Mulberry, seed and fruit are limited to those individuals bearing pistillate flowers. From this it might appear that the stamens take no part in the work of pro- ducing the seed; but observations show that unless stamens are close at hand, the pistil will produce no seed. A well isolated Corn plant with tassel removed before FIG. 35. — Flower of the Cherry the stamens are mature will pro- with Parts of fche Pistil indicated.

duce no kernels. Some varieties °> °7ary1; *' stigma; «' style*

f 0, , ,. , Much enlarged,

of Strawberries are dioecious, and

unless both kinds of plants are grown in the same bed, there will be no seed or fruit.

To understand just how the essential organs function in seed production, a careful study of their parts must be made.

Parts of the Pistil. — The pistil usually consists of three parts: the enlarged base which is the ovary and the portion in which the seeds develop; the flattened or expanded surface at the upper extremity, known as the stigma; and the stalk-like part connect-

33

34

PISTILS AND STAMENS

ing the ovary and stigma, known as the style. In the pistil of the Cherry shown in Figure 85 the parts are indicated. The ovary is at o. The stigma is the expanded surface at st. The style is at s and is a stalk-like structure projecting from the ovary and supporting the stigma.

In the Corn the style is extremely long and the stigma branched. (Fig. 36.) In Wheat, Oats, Barley, and Rice there are two very short styles and the stigmas are much branched and plume-like. (Fig. 87.) Styles and stigmas vary much among plants.

Ovary. — The ovary is the most impor- tant part of the pistil because within it the seeds are produced, and often it makes the edible portion of fruits.

— st

FIG. 36. — Pistillate flower of Corn, drawn to show the parts of the pistil. A portion of the bracts have been cut away to give a view of the ovary, o, ovary, the portion that becomes the ker- nel; s, style; st, stigma. Much enlarged.

FIG. 37. — Pistil of Wheat and the two lodicules. o, ovary; st, stigmas; s, styles; Z, lodicules. Much en- larged.

FIG. 38. — Cross section of the ovary of a Tomato. o, ovary wall; b, partition walls of the ovary; c, locules or cavities in the ovary; d, ovules; p, placentas or parts of the ovary to which the ovules are attached. Much enlarged.

When the 'ovary is sectioned so that its interior may be studied, it is seen that it is not a solid body, but consists of a wall enclosing one or more cavities, called locules. (Fig. 38.) In these cavities or locules are the small bodies called ovules, each of which is capable of developing into a seed. Point out the parts of the ovary shown in Figure 38.

The ovary may contain one locule or many and the number of ovules in a locule also varies in different ovaries. In Beans and

OVARY

35

FIG. 39. — Flower and pod of the Garden Pea. A, section through the flower to show ovules, a, ovary; o, ovules; 6, stamens; t, stigma; s, style. B, the matured ovary, called pod, opened to show the matured ovules or seeds (e). Flower enlarged but pod less than natural size.

FIG. 40. — A, pistil of Red Clover with one side of ovary cut away so that the ovules (o) may be seen, a, stigma; s, style. B, lengthwise section through the ovary and ovules of Red Clover and very much enlarged to show the parts of the young ovules, w, ovary wall; o, ovules; s, base of style; st, stalk or funiculus of the ovules; n, nucellus; i, integuments.

36

PISTILS AND STAMENS

Peas, the ovary has one locule enclosing a number of ovules. In A of Figure 39, showing a lengthwise section through the flower of the Pea, one side of the ovary wall is removed to show the locule with its ovules. In this particular flower of the Pea, there are six ovules, but other flowers might have more or fewer. In B of Figure 39 is shown the ovary after it becomes a mature pod. The pod is opened to show the seeds. Each /> seed is a developed ovule and the pod enclos-

ing the seeds is the ovary wall much enlarged. Notice how the ovules and seeds compare in number.

In Red Clover, shown in Figure 40, there is one locule and two ovules. The ovaries of Alfalfa have only one locule, but may have as many as eighteen ovules.

In the ovary of Corn, Wheat, Oats, and Grasses in general, there is one locule and a

FIG. 41. — Length- wise section through a young pistil of Corn to show the locule and ovule, a, ovary; s, style; o, ovule consisting of nucellus (n) and integuments (i); I, locule or cavity in which the ovule is located. Much en- larged.

•FiG. 42. — Lengthwise section through a Tomato flower to show the interior of the ovary, a, ovary; I, locules, represented by dark shading; o, ovules; p, placentas. Much enlarged.

single large ovule. A lengthwise section through the pistil of Corn is shown in Figure 41- Notice the ovule at o and that it almost fills the locule.

Tomato ovaries have few or many locules which contain a large number of ovules. Figure 4@ shows a lengthwise section of a Tomato ovary showing two locules and many ovules. By count-

SIZE OF OVULES 37

ing the ovules shown in Figure J^2 and those shown in Figure 38 the number of ovules in a Tomato may be roughly estimated.

An examination of the ovaries of many plants would show considerable variation in the number of locules and ovules, but in general, all ovaries consist of an ovary wall enclosing one or more locules which contain one or more ovules.

Ovule. — Since ovules develop into seeds, they have the most to do with seed production and are, therefore, the most directly related to the function of the flower. The process of fertilization, one of the most important events in plant life, takes place in the ovule and a good understanding of fertilization requires a knowl- edge of the ovule.

Size of Ovules and how their Number Compares with the Num- ber of Seeds. — Although ovules are the chief structures in per- forming the function of seed-production, in size they are usually very inconspicuous and not much can be learned about them without the aid of the microscope. a

In many plants the ovules are barely visible to [

the unaided eye. When ovaries and ovules are @

shown in drawings, they are usually much en- larged, so that much more is shown than could , , ,IG' ' ~

of the Tomato taken be seen by cutting sections and studying the from the flower and

ovaries themselves, unless a microscope were drawn natural size. used. In Figure 4$, the pistil of the Tomato is shown natural size. By comparing it with the pistil shown in Figure 4@, it will be seen that in order to show the structures of the ovary, the pistil in the latter Figure is much enlarged.

Since ovules are small, it is difficult to count them in ovaries where they are numerous. It is possible in many cases to make a rough estimate of the number of ovules by counting the seeds produced. Since each seed is a developed ovule, there must occur in the young ovary as many ovules as there are seeds in the mature ovary. From this it follows that those Tomatoes con- taining two hundred or more seeds must have had as many ovules in their young ovaries.

If all the ovules became seeds then a count of the seeds would give the exact number of ovules ; but in many cases, due to a lack of fertilization, space, or sufficient food supply, only a part of the ovules complete their development and become seeds. In Red Clover, as shown in Figure 40, there are two ovules, but when the

38

PISTILS AND STAMENS

mature pod is threshed, only one seed is found. In Alfalfa only about one third of the ovules produce seed. In the Apple, Pear, Tomato, and other fruits some of the ovules often fail to develop, and in case of seedless fruits none of the ovules complete their development. In most fruits the production of seed is not an important feature to the plant grower, the seedless fruit in many cases being more desirable; but in case of Clover, Alfalfa, Flax, and other plants valuable for seed, the value of the plant as a seed producer is directly related to the number of ovules which be-

Ji B

FIG. 44. — Surface view of an ovule at two stages of development. A, stage of development showing the integuments (a, 6) growing up over the nucellus (n). B, older stage in which the integuments have closed over the nucellus, leaving only a small opening, the micropyle (ra) . s, the funiculus. Much enlarged.

FIG. 45. — Section through the ovule of Red Clover show- ing the embryo sac. em, em- bryo sac with the egg (e) and the primary endosperm nucleus (en) indicated; i, integuments; m, micropyle. Many times enlarged.

come seed. How much could the seed yield of Clover and Alfalfa be increased if they could be made to develop all of their ovules into seed? If clover seed were selling at $10 per bushel, what would be the value of the increased yield on ten acres of average Clover?

Parts of the Ovule. — The ovule consists of a main body and a stalk known as the funiculus which connects to the ovary wall. The main body consists of a central (usually rounded) portion called nucellus, which is enclosed by one or more coverings called integuments that grow up from the funiculus. In Figure 40, showing the ovules of Clover, the stalk or funiculus is at st; the central portion or nucellus of the main body is at n; the coverings or integuments of the nucellus are at i. Turn to this Figure and point out these parts. In the ovule of the Corn, shown in Figure 41, the funiculus is apparently absent. In Figure 44 is shown a

HOW THE PARTS OF AN OVULE ARE MADE UP 39

surface view of an ovule at two stages of development. Notice how the nucellus is enclosed by the integuments, leaving only a small opening at m known as the micropyle.

The pollen tube, a tube-like structure produced by the pollen grain in connection with fertilization, often uses the micropyle as an entrance to the ovule. Some ovules are straight but oftener there is a curving to one side during growth as shown in Figure 44- By curving the micropyle is brought near the base of the ovule, a position more favorable for the entrance of the pollen tube.

How the Parts of an Ovule are made up. — The ovule, like all other parts of the plant, is made up of many living units called cells. A cell consists of a mass of living matter called protoplasm, which is generally enclosed by walls. A very important part of the living matter is the nu- cleus, a globular body commonly occupying a central position in the cell. The ovule, although a very small body, is composed of many hundreds of cells, all of which are in some way related to seed formation.

The cells of the funiculus, in- teguments, and most of those of

the nucellus furnish food and de- thr°ugh the ovary °.f Corn sh<ring

. embryo sac. o, ovule; em, embryo

velop a covering for the inner and sac; 6) egg; eri) the two nuclei which

more vital parts of the seed. In fuse to form the primary endosperm form and structure they are nucleus; i, integuments; w, ovary similar to cells composing other wall; s, base of style or silk. Much parts of the plant. The cells enlar^ed-

peculiar to the ovule are those forming a special group, usually seven or eight in number and occupying a central position in the nucellus One peculiar feature of these cells is that they usually are not separated by cell walls and their masses of protoplasm lie in contact or closely join with each other. The region which these cells occupy is known as the embryo sac, so named because within it the embryo develops. The embryo sac, being deeply buried in the nucellus wh ch is in turn enclosed by the integu- ments, is well protected and to study it the ovule must be sec-

FIG. 46. — Lengthwise section

40

PISTILS AND STAMENS

tioned. In some ovules the embryo sac may be seen without the microscope, but in most ovules it is microscopic. There is only one cell and one nucleus in the embryo sac, which have an important function in the formation of the seed. The important cell is the egg. The egg is at the micropylar end and after fertilization produces the embryo of the seed. The important nucleus, referred to as nucleus because it has no definite amount of protoplasm, is the primary endosperm nucleus. It is near the center of the embryo sac and is important because upon it the development of the stored food or endosperm of the seed depends. The remaining cells and nuclei of the

FIG. 47. — A vertical section through an Oat ovary to show the parts of the ovule. Parts of the lemma, palea, and two stamens are shown, and one style and stigma remains. Label the parts of the ovule. Much enlarged.

embryo sac are absorbed and disappear soon after the egg is fertilized. In the ovules of Clover and many other plants, the cells at the inner end (chalazal end) of the embryo sac disappear even before the egg is fertilized.

A section through an ovule of Red Clover is shown in Figure 45. Point out the embryo sac. Notice the egg at e and the endosperm nucleus at en. Point out the embryo sac of Corn in Figure 46. Notice that instead of a single primary endosperm nucleus, there are two nuclei lying in contact. These nuclei fuse and form the primary endosperm nucleus. A section through an ovule of Oats is shown n Figure Jtf. Point out the embryo

THE POLLEN GRAIN AND ITS WORK 41

sac, egg, and primary endosperm nucleus. Redraw this figure on a sheet of paper and label the parts.

Although pistils vary much in number of carpels, length of styles, and in number of locules and ovules, there is uniformity in organization and adaptation of parts to special functions. The stigma is especially adapted for receiving pollen, the style supports the stigma in a position suitable for receiving the pollen, and the ovary protects the delicate ovules in which is the embryo sac containing the egg and primary endosperm nucleus, which are the chief structures of the pistil.

The Stamen. — The stamen usu- ally consists of two parts; the en- larged terminal portion, or anther; and the stalk, or filament. The filament is often so short as to seem to be absent. Point out the parts

of the stamen in A of Figure 48.

a, an-

ch en- an anther,

called locule, which contains many showing the locules and pollen globular bodies known as pollen or grains. The two locules at the pollen grains. When the pollen is lef^ ka™es°pened' allowing the mature, the walls of the anther *

open and allow the pollen to escape. Notice the cross sec- tion of an anther shown in B of Figure 48- Point out the locules and pollen grains. Notice that two of the locules have opened.

The Pollen Grain and its Work. — The pollen grain is a cell with its living matter enclosed in a heavy protective wall. It needs to be well protected, for during its journey to the pistil, destructive agencies such as cold, heat, and drying are encoun- tered. The transference of the pollen to the stigma is called pollination. Pollination is a very important event, for the pollen cannot perform its function except on the stigma.

On the stigma the pollen grain grows a tube which traverses the stigma and style, pierces the ovule, and reaches the embryo sac. Pollen grains, when first formed in the anther, have only one nucleus, but in preparation for the work of fertilization, there is nuclear division and as a result there are three nuclei in a well

r^\^ j.i_ 11 £ 111 . . — ,

The anther is usually four lobed ther; ^ ^^^ ^

and within each lobe is a cavity, larged cross section of

42

PISTILS AND STAMENS

developed pollen tube. This feature is shown in Figure 49. The nucleus at the end of the tube and known as tube nucleus directs the growth of the tube and disappears soon after reaching the embryo sac. The two nuclei following closely be- hind the tube nucleus are the sperms or male nuclei, the structures which join with the egg and primary endo- sperm nucleus in fertilization. The pollen tube is a passage way through which the sperms pass to the embryo sac.

Fertilization. — After the two sperms reach the embryo sac, one approaches the egg and fuses with its nucleus, while

FIG. 49. — Pollen grains in different stages preparatory to fertilization. A, surface view of a pollen grain; B, section through pollen grain in uni-nucleate stage; C, section through pollen grain showing the nucleus divided into the generative (g) and tube nucleus (<); /), pollen tube forming into which the two nuclei have passed; E, tube more developed and generative nucleus divided into two sperms (g). Much enlarged.

FIG. 50. — A diagram of a length- wise section through the pistil of Red Clover, showing pollen tubes trav- ersing the stigma and style. Two pollen tubes have reached the em- bryo sacs, p, pollen grains develop- ing tubes; st, stigma; p.t, pollen tubes; o, ovules; e, egg; en, en- dosperm nucleus; s, sperms. Much enlarged.

the other approaches the primary endosperm nucleus and fuses with it. This process of fusion is called fertilization. Since there are two

THE DEVELOPMENT OF THE OVULE INTO A SEED 43

fusions, there are two fertilizations, and the two fertilizations are called " double fertilization." Both egg and primary endo- sperm nucleus are now said to be fertilized, and the pollen grain has performed its function, which is an important one, for with- out fertilization the ovule would not develop into a seed. Pollination, the growth of the pollen tube to the embryo sac, and the formation of the two sperms are simply preliminary acts to fertilization, which is the final achievement of the pollen

grain. Study the pollen grains shown in Figure 49. Notice that the tube has broken through the

FIG. 51. — Stigma of Corn show- ing how the pollen grains grow their tubes into the stigma, p, pollen grains; t, pollen tube. Much enlarged.

FIG. 52. — A, diagrammatic section of an ovule of the Tomato in which the egg (6) and primary endosperm nucleus (d) have been fertilized, o, portion of ovule surrounding"1 and en- closing the embryo sac. B, diagram- matic section of the seed of the Tomato, e, embryo; c, endosperm; t, seed coat. The lines drawn from the ovule to the seed indicate the parts of the ovule from which the different parts of the seed have de- veloped. Both are enlarged but the ovule is enlarged much more than the seed.

pollen wall. How have the two sperms been formed? In Figure 50 trace the pollen tubes to the embryo sac. How do the pollen tubes make their way through the style? Where do they obtain their food for growth? Notice how the pollen tubes enter the branched stigma of Corn in Figure 51.

The Development of the Ovule into a Seed. — After the egg and primary endosperm nucleus have been fertilized, the ovule begins its development, which results in the production of a seed. There

44

PISTILS AND STAMENS

are three main structures involved in this development: (1) the fertilized egg; (2) the fertilized primary endosperm nucleus; and (3) the parts of the ovule surrounding the embryo sac. The development of each of these parts into their respective seed parts takes place simultaneously. The fertilized egg becomes the embryo, the endosperm nucleus has to do with the forming of the endosperm, and a part of the surrounding portion of the ovule becomes the seed coat. Figure 52 shows a Tomato ovule

-w

FIG. 53. — A young ovary of Corn just after fertilization and a mature ovary or kernel, both of which are sectioned lengthwise and the relation of parts indicated. A, lengthwise section of the young ovary showing nucellus (n), egg (e), endosperm nucleus (en), integuments (i), ovary wall (w), and base of style (6). B, the lengthwise section through the kernel showing the embryo (em), endosperm (end), seed coat (c), ovary wall (w), and the base of the style (6) . The dotted lines indicate the parts of the ovule from which the different parts of the kernel have developed.

in which the egg and endosperm nucleus have just been fertilized and also shows the seed which develops from the ovule. The lines indicate the parts of the ovule from which the different parts of the seed have come. Study Figure 53 showing the development of the ovule of Corn into a seed. Point out the different parts of the kernel and the part of the ovule from which they came. Notice that the heavy outer covering of the kernel is the ovary wall, and does not come from the ovule. A kernel of Corn is a seed closely jacketed by the ovary wall. Copy on a sheet of

THE DEVELOPMENT OF THE OVULE INTO A SEED 45

FIG. 54. — A, a vertical section through an Oat ovary showing one style and stigma, the ovary wall, and the parts of the ovule. B, a vertical section through an Oat kernel showing its parts. After comparing with Figure 53 label the parts of A and B and with lines indicate the parts of A from which the parts of B have developed.

FIG. 55. — A diagram showing the relation of the parts of the ovule to those of the seed in Red Clover. A, ovule just after fertilization showing the egg (e) and the endosperm nucleus (d). B, seed with half of the seed coat (s) removed to show the large embryo (em). The dotted lines indicate the relation of the parts of the ovule to those of the seed.

46 PISTILS AND STAMENS

paper the drawings in Figure 54 and with lines indicate the parts in A from which the different parts shown in B have come.

In many plants the endosperm does not remain outside of the embryo as it does in Corn and other grains. If one removes the thin rind-like testa from a soaked Bean, all that remains is the large embryo. The endosperm is stored in the embryo and as a result the embryo is much enlarged and fills the space within the testa. Clover, Alfalfa seed, and many other seeds have the endo- sperm stored in the embryo. Study the Clover seed in Figure 55. Notice that there is apparently no endosperm, and that the .much enlarged embryo occupies nearly all the space within the testa.

In some seeds a stored food known as perisperm occurs. Usually as the ovule develops into the seed, the nucellus is de- stroyed and replaced by the developing endosperm, leaving only the integuments from which the seed coat is formed. However, in the formation of a few seeds, some of the nucellus remains, and a portion of its outer region becomes filled with stored food, thus forming the layer of stored food known as perisperm, which sur- rounds the endosperm and embryo.

Pollination

Nature of Pollination. — Pollination is the transference of pollen to the stigma. After the pollen is on the stigma, it may produce a tube reaching to an ovule and effect fertilization, or it may lie dormant; but in either case the stigma is considered pollinated. Much pollination occurs in nature that does not result in fertilization. Corn pollen, for example, as it is blown about may fall on the stigmas of various other species of plants, but since no fertilization results, the pollination is not effective. Pollen is usually effective only on stigmas of plants similar to the plant which produced the pollen. Thus Apple pollen is effective only on Apple stigmas, Corn pollen only on Corn stigmas, etc.

Pollinating Agents. — The most important pollinating agents are gravity, wind, insects, and man. In some cases, as in Rice, Wheat, and Oats, where the pollen falls from the anthers to the stigma, pollination depends upon gravity. Even in orchards some pollination may be accomplished by pollen falling from the higher branches. x In early spring, before there are many insects, many of our trees, such as Willows, Poplars, Oaks, and Pines,

KINDS OF POLLINATION 47

depend upon the wind for pollination. The wind is also an important agent in the pollination of Corn and aids some in orchard pollination. Plants having showy flowers depend upon insects for pollination and it is among these plants that attractive colors, secretions of nectar, and various structural arrangements, which are interpreted as adaptations to secure pollination, occur. The pollination of Fruit trees, Clovers, and Alfalfa is done chiefly by insects. (Fig. 56.) In experimental work, such as crossing

FIG. 56. — Bumble bee pollinating Red Clover.

Tomatoes, Corn, and Fruit trees, man himself often does the pollinating so as to have it under control.

Kinds of Pollination. — On the basis of the relation of the stamen furnishing the pollen to the pistil pollinated, there can be different kinds of pollination. The transfer of pollen from the stamen to the pistil of the same flower is self-pollination, while the transfer to the pistil of another flower is cross-pollina- tion. Various relationships may occur in pollination. Thus the

48 PISTILS AND STAMENS

pistil of a Ben Davis Apple blossom may be pollinated: (1) with pollen from the same flower; (2) with pollen from another flower in the same cluster; (3) with pollen from a flower on another branch; (4) with pollen from another Ben Davis tree located in the same or a neighboring orchard; or (5) with pollen from a Jonathan or some other different variety. In case of fruit trees horticulturists sometimes consider the pistil of a blossom self- pollinated if the pollen comes from the same flower, from another flower on the same tree, or from another tree of the same kind, and consider the pistil cross-pollinated only when the pollen comes from another variety of fruit tree. Corn breeders speak of self-, close-, and cross-pollination. Pollination resulting from the pollen falling from the tassel to the silks of the same plant is called self-pollination. Pollination in which the pollen from one plant falls on the silks of another plant is called close-pollination if both of these plants came from kernels taken from the same ear, but cross-pollination if these plants came from kernels taken from different ears. In case of cross-pollination, the plants may be of the same variety or of different varieties.

The Amount of Pollen Required for Good Pollination. — One pollen grain is required to fertilize each ovule, and, therefore, a pistil with many ovules requires many pollen grains for good pollination. In Corn, Wheat, and Oats where there is only one ovule, one good pollen grain on the stigma is sufficient, although a large number is usually present. Due to the great waste of pol- len during transportation, much more is produced than is really needed. A medium-sized plant of Indian Corn produces about 50,000,000 pollen grains or about 7000 for each silk. Many of these never reach a silk, and of the many that do all, except the one that reaches the ovule first with its tube, accomplish nothing. On the stigma of the Red Clover, although each pistil has only two ovules, there are often as many as 25 pollen grains, 23 of which are wasted.

On the other hand, in flowers where the ovaries contain numer- ous ovules, as in Tomatoes and Melons, it often happens that not enough pollen reaches the stigma to effect fertilization in all the ovules. In the Tomato, for example, an ovary may contain as many as 200 ovules, in some of which fertilization may not occur because of insufficient pollination. Even in Beans, Apples, and Pears, where the ovules are not numerous, one often finds in

HOW POLLEN IS AFFECTED BY EXTERNAL FACTORS 49

the mature fruit some undeveloped ovules, which due to the lack of fertilization did not become seeds. Although much of the vari- ation that occurs in the number of seeds in many of the fruits is due to the failure of the pollen to function properly on the stigma or to the insufficient nourishment of the ovules, much of the vari- ation can be attributed to insufficient pollination.

There is good evidence that the imperfect development of fruit is due in some cases to insufficient pollination. By polli- nating the stigmas of Tomatoes in such a way that portions of the stigmas received no pollen, one 1 investigator found that no fertilization occurred in some locules, and that the portion of the ovary surrounding these locules developed much less than those portions of the ovary surrounding those locules in which fertili- zation occurred, thus causing one-sided fruits.

How Pollen is Affected by External Factors. — Pollen is not so specially prepared as seeds are to endure extreme conditions during transportation. During transportation and while on the stigma, pollen may be either killed or rendered functionless by extremes of temperature and moisture. The pollen of most plants is so sensitive to dryness that an exposure to the ordinary dryness of the air cannot be endured more than a few days and in many cases only a few hours.

In the storage of pollen, which is sometimes necessary in experi- mental work, the main caution is to store the pollen where it will not be dried out too much by evaporation, although the pol- len must be kept dry enough that it will not mold. It has been found that Plum and Apple pollen can be kept alive much longer when stored in closed chambers where there is less drying than in laboratory air. One investigator has reported that Corn pol- len will die in two or three hours when exposed to the air of the laboratory or living room, but will live two days when stored in a moist chamber. Some investigators think that hot dry weather during the pollination of fruit trees may affect the setting of fruit by destroying some of the pollen.

The pollen of some plants, as in case of Red Clover and Alfalfa, absorbs water so rapidly that it is destroyed by bursting when immersed in water or stored in a saturated air. Consequently these plants are not successfully pollinated when they are wet

1 Pollination and Reproduction of Lycopersicum esculentum (Tomato). Minnesota Botanical Studies, p. 636, Nov. 30, 1896.

50 PISTILS AND STAMENS

with dew or rain. Apple pollen and the pollen of many other fruit trees, although not destroyed when immersed in water, will not function nearly so well and for this reason rain or dew on a stigma may hinder the pollen in its work.

The pollen of many plants is quite sensitive to a low tempera- ture, showing a decrease in vitality when exposed for a few hours to a temperature only a little below freezing. Pollen, if not in- jured by cold, will not germinate while the temperature is low. In the Apple, Pear, Plum, Peach, and Cherry l a temperature of — 1°C. has been found to interfere with the proper functioning of the pollen by injuring the stigmas and preventing the ger- mination of the pollen. Cold during the blooming period may be responsible for much failure in fruit-setting.

The Results of Pollination. — The most immediate as well as the most important result of pollination is the fertilization of the egg cell and primary endosperm nucleus. Through the process of fertilization the pollen stimulates the ovule and other struc- tures to develop, and transmits factors by means of which the embryo and the endosperm of the seed inherit the characters of the pollen parent.

The importance of the stimulative effect of fertilization in the development of a seed is obvious, for unless fertilization occurs, the egg, endosperm nucleus, and other parts of the ovule rarely develop into their respective seed structures, and con- sequently the ovule either disappears or remains as a small withered body as often seen in fruits. Furthermore, the devel- opment of fruit depends upon the stimulative effect of fertiliza- tion, as shown in case of fruit trees, Melons, Alfalfa, etc., in which the flowers wither and fall from the plant unless fertilization occurs in some of the ovules. There are, however, a few instances in which the stimulative effect of fertilization is not necessary, as in seedless Oranges, seedless Persimmons, Bananas, and a few other fruits known as parthenocarpic fruits, which develop, although no fertilization occurs. There are a few plants, the Dandelion being a common one, in which ovules develop into seeds parthenogenetically, that is, without fertilization, but such plants as well as those that develop seedless fruits are exceptional. In most cases our harvest of seed and fruit depends upon the stimulative effect of fertilization.

1 Research Bulletin 4, Wisconsin Agr. Exp. Sta., 1909.

THE RESULTS OF POLLINATION

51

The effect of fertilization in reference to the influence which the sperms have upon the character of the endosperm of the seed and upon the character of the plant which the embryo of the seed will produce is a sub- ject receiving much attention in plant-breed- ing. The endosperm nucleus consists of a sperm and a primary endosperm nucleus, each of which is capable of determining the character of the endosperm. Likewise, in the fertilized egg, the contents of both sperm and egg are capable of determining the characteristics of the plant developing from the fertilized egg. But the influence of the sperm is toward the production of both endosperm and plants having the features which are characteristic of the pollen parent, while the egg and primary endosperm nucleus tend to reproduce in the offspring those features characteristic of the mother plant. Thus it follows that if the pollen parent is very different from the mother plant, as is the case when the parents be- long to different varieties or species, there will be opposing tendencies in the fertilized egg and endosperm nucleus. Such a fertil- ized egg develops into a plant known as a hybrid. The hybrid character of the endo- sperm in most seeds is either lost through Sweet Corn showing the absorption of the endosperm by the the effect of the pollen

embryo or obscured by coverings. It is in of, Yellow Dent Co™- ,, /; £ , . ~ , The plump kernels

the Grass type of seeds, as in Corn where have endosperm like

the endosperm remains outside of the em- the Yellow Dent Corn, bryo and can be seen through the pericarp, due to the influence of that the influence of the sperm on the the sperm which fused endosperm and known as xenia is often with the primary endo- ..ul AT ,. ,,- £ ^ , sperm nucleus. After

noticeable. Notice the ear of Corn shown H j ^ebber

in Figure 57. This was an ear of Sweet Corn which was partly pollinated with pollen from hard Field Corn. Notice the kernels which have the hard plump endo- sperm and resemble the kernels of Field Corn. In the de-

FIG. 57. — An ear of

52

PISTILS AND STAMENS

velopment of these kernels, the sperm portion of the endo- sperm nucleus dominated, and thus the endosperm is like the endosperm of the pollen parent. The sperm may even deter- mine the color and fat content of the endosperm. On the other hand, if Field Corn is pollinated with pollen from Sweet Corn, then usually the primary endosperm nucleus dominates and one sees no effect of the sperm. Thus it is seen that the character of the endosperm of a seed may be determined by either of the members which fused in forming the endosperm nucleus.

The kernels in Figure 57 which have the endosperm features of Field Corn also have embryos with opposing tendencies.

FIG. 58. — Pears showing a difference between the results of self- and cross- pollination, a, fruit resulting from self-pollination; b, fruit resulting from cross-pollination. After Waite.

These embryos received from the egg tendencies to develop into plants having all of the features of Sweet Corn. They also re- ceived from the sperm tendencies to develop plants having all of the features of Field Corn. In the hybrid offspring it is likely that some of the characters of both parents will be present.

The Kind of Pollination Giving the Best Results. — Plants in general seem to favor cross-pollination and often have their flowers so constructed as to prevent self-pollination. In some plants, however, as in the small grains, Beans, Peas, and some other plants, self-pollination is the usual method and gives good results. Red clover, many fruit trees, and many other plants require cross-pollination and will develop very little seed or fruit

THE KIND OF POLLINATION GIVING BEST RESULTS 53

when self-pollinated. Many of our Pears, such as the Anjou, Bartlett, Pound, Lawrence, Jones, Howell, Sheldon, Wilder, and some others will not produce much fruit unless pollinated with pollen from other varieties, while the Kiefer, Buffum, Seckel, and some others known as self-fertile varieties set fruit well when self -pollinated. Moreover, some trees which are self-fertile develop larger and better fruit when cross-pollinated. (Fig. 58.) Many of our Apple trees and Cherry trees are known to require cross-pollination . Furthermore, some varieties of fruit trees l which require cross-

FIG. 59. — Results of cross-pollination with different varieties in the Sweet Cherry. A, fruit obtained by pollinating a cluster of flowers of the Bing with pollen from the Black Republican. B, fruit obtained by polli- nating a cluster of flowers of the Bing with pollen from the Knight. After V. R. Gardner.

pollination will not do equally well when crossed with all varieties. In Apples, Pears, and Cherries better results have been obtained

1 The pollination of pear flowers. Bulletin 5, Div. of Veg. Path., U. S. Dept. of Agr., 1894.

Pollination of the apple. Bulletin 104, Oregon Agr. College Exp. Sta., 1909.

Pollination of the Sweet Cherry. Bulletin 116, Oregon Agr. College Exp. Sta., 1913.

Read Pollination in Orchards. Bulletin 187, Cornell University Exp. Sta., 1909. Also Pollination of Bartlett and Kiefer Pear. Ann. Report, Virginia Agr. Exp. Sta., 1911.

54 PISTILS AND STAMENS

by crossing with some varieties than with others. (Fig. 59.) In case of Sweet Cherries, when flowers of the Bing, a variety requir- ing cross-pollination, were pollinated with pollen from the varietj^ called the Knight, only a few fruits developed; while flowers pollinated with pollen from the Black Republican produced fruit abundantly. Obviously much of the success in orcharding has to do with securing for each variety of fruits the best kind of pollination.

CHAPTER V

SEEDS AND FRUITS

Nature and Structure of Seeds

The seed is the principal structure by which plants increase in number. The chief function of a seed is to produce a plant like the one that bore it. For plants to increase in number and at the same time thrive well, they must spread to new areas. Seeds are thus so constructed that they can separate from the parent plant and be carried to regions where there is opportunity for new plants to develop. Seeds, being able in a dormant state to live long and endure adverse conditions, are the means by which those plants living only one season are able to perpetuate themselves. As to origin the seed is sometimes defined as a matured ovule, that is, it is an ovule in which three things have taken place: (1) the fertilized egg has developed into an embryo, the miniature plant of the seed; (2) the fertilized primary endo- sperm nucleus with some adjacent protoplasm has produced a mass of stored food or endosperm; and (3) the outer portions of the ovule have been modified into a testa or seed coat. Despite a wide variation in size, shape, color, and other external features, seeds possess in common an embryo, stored food, and seed coat. In many cases these three parts are not separate, for the endo- sperm may be absorbed by the embryo during the development of the seed. This is true in the Bean, Pumpkin, and a number of other families, where the seeds consequently have only two distinct parts, embryo and testa.

Each part of the seed has a distinct function to perform. The embryo develops into a new plant, the reserve food nourishes the young plant until roots and leaves are established, and the seed coat protects the embryo and endosperm during the resting stage of the seed. It is due to the embryo that seeds are valuable in the production of new plants, while the stored food makes many seeds valuable food for animals.

The embryo, which is the chief structure of the seed, is the

55

56

SEEDS AND FRUITS

young plant, which after reaching a certain stage of development, varying in different plants, passes into a dormant stage from which it may awake if conditions are favorable and continue its devel- opment until it becomes a mature plant. In the development of the embryo from the fertilization of the egg to the dormant stage, certain structures which function in the further development of the young plant are usually more or less developed. In a well formed embryo like that of the Bean, there are four parts, hyocotyl, plumule, cotyledons, and radicle. In Figure 60 of the Bean, h is hypocotyl, p, plumule, and c, cotyledons. The radicle (r) is at the lower end of the hypocotyl and is so closely joined with the

hypocotyl that it does not appear as a separate structure. The cotyledons of the Bean have absorbed the endo- sperm and consequently are so much enlarged that they form the bulk of the embryo. The special functions performed by the different parts of the embryo are quite noticeable in the germination of the seed. The cotyle- dons supply food; the plumule develops stem and leaves; the radicle develops a root; and the hypocotyl in many cases pulls the cotyledons and plumule out of the seed coat and raises them above ground.

The stored food and seed coat are temporary structures. They nourish and protect the young plant in its early stage of develop- ment and then disappear. The stored food, consisting chiefly of starch, proteins, and oils, the proportion varying in different seeds, develops in close contact with the embryo and when not absorbed as rapidly as it develops, it forms the storage tissue or endosperm in which the embryo becomes imbedded. The testa, the protective structure of the seed and usually formed from the integuments of the ovule, generally consists of a single covering so much thickened and hardened that it protects the embryo against injuries. Often there is a thin inner covering and in exceptional seeds, like those of the Water Lily, an extra outer covering called the aril develops later than the integuments and forms a loose covering about the seed. (Fig. 62.)

FIG. 60. — Bean with testa removed and cotyledons spread apart, c, cotyledons; h, hypocotyl; p, plumule; r, radicle.

NATURE AND STRUCTURE OF SEEDS

57

On the surface of seeds occur certain structures which suggest the structural relation of the seed to the ovule. The micropyle, the small opening through which the pollen tube entered the ovule, persists as a tiny pit on the seed coat. Usually near the micropyle there is a much larger scar, called the hilum, left where the seed broke away from the funiculus, the stalk-like structure which attached the ovule to the ovary and through which the seed received food and water during its development. (Fig. 61.) In case an ovule turns over on its elongated stalk and grows fast to it, the stalk persists on the seed coat as a distinct ridge, called the raphe. (Fig. 62.) In some seeds, like those of

FIG. 61. — Beans showing the hylum at h and the micropyle at m.

A B <<'

FIG. 62. — A, seed of Pansy showing raphe (r) . B, seed of Castor Bean show- ing caruncle (c). C, seed of White Water Lily showing the aril or loose jacket around the seed.

the Castor Bean, an enlargement known as the caruncle develops near the micropyle.

Structures such as hairs, plumes, hooks, and other appendages which do not occur on ovules, are direct outgrowths of the seed coat and function chiefly in dissemination. Similar appendages occur often on one-seeded ovaries in which case one can tell only by dissection whether the structure is a seed or one-seeded fruit.

Many of the small one-seeded fruits are commonly called seeds. In addition to a seed, they contain the ovary wall which persists as an outer covering over the seed. The so-called seeds of Let- tuce, Buckwheat, Ragweed, and the grains such as Corn, Wheat, Barley, Rye, and Oats are familiar examples of one-seeded fruits which are commonly called seeds. While they are not identical with true seeds in structure, they are in function and therefore may be appropriately discussed with seeds. In these one-seeded fruits, the seed is protected by .the hardened ovary wall, and consequently, the seed coat is poorly developed, forming only a

58 SEEDS AND FRUITS

thin covering, which is usually .tightly pressed against the inner side of the ovary wall.

In general structure seeds are similar, all having an embryo, stored food, and seed coat, but in size, shape, and in features which pertain to the structure of the embryo, composition of the stored food, and character of the seed coat, seeds vary widely and can be used in many ways by man. The number of coty- ledons developed by the embryo is used as a basis upon which to classify the Flowering Plants into two classes, Monocotyledons and Dicotyledons. From the stored food, whether stored as endosperm or in the embryo, various valuable products, such as starch, protein, fats and oils, are obtained; and from the hair- like outgrowth of the seed coat, as in case of Cotton, various fiber products are made. Although seeds may be divided into many types on the basis of their structure and external features, only those types which include the most common seeds will be studied in this presentation.

Bean Type of Seeds. — Of this type of seeds, those of the Bean, Pea, Peanut, Clover, Vetch, Alfalfa, Cotton, Pumpkin, Squash, Melon, Apple, Peach, Oak, Hickory, and Walnut are ex- amples. The type is so named because it is characteristic of the Bean family (Leguminosae) , a family notable for its many valu- able cultivated forms among which are Clover, Alfalfa, Beans, Peas, Vetch, and Peanuts. The type is also characteristic of the Rose family (Rosaceae) , the family to which most fruits, such as the Apple, Peach, Pear, Cherry, etc. belong. In this family, however, it is the fruit (rarely the seed) that is important. The seeds of the Bean type are common to a number of plant families and to species and varieties of plants so numer- ous that a list naming them all would require a page or more. Although many are valuable commercial seeds, some are borne by weeds and hence of interest because of their undesirable features.

These seeds differ from other types in having little or no endo- sperm. As the seed develops, all or almost all of the food tis- sue formed by the endosperm nucleus and adjacent cytoplasm is absorbed by the embryo where it is stored in the cotyledons, which, consequently, are so much enlarged that they are much the largest part of the embryo. (Fig. 63.) For this reason these seeds are called exalhuminous seeds, that is, seeds without

SEEDS OF THE BUCKWHEAT AND FLAX TYPE

59

-e-

FIG. 63. —4, Squash

endosperm. Another feature to be noted is that the embryo has two cotyledons.

In external characters they vary so much that their type in most cases can be determined only by an examination of their structure. In size, those most commonly grown in our region vary from the smallest of the Clover Seeds up to the largest of the Beans. They are kidney-shaped, glob- ular, oval, or flattened. Among them vari- ous colors such as red, purple, brown, yellow, green, mottled, and black occur. In identifying the different seeds of this type, especially those of the Bean family, size, shape, and color are important aids.

In importance, the seeds of this type rank next to those of the Grass family. In Beans, Peas, and Peanuts, which are used directly as food, the value depends upon

the protein, fats, and starches stored in the seed sectioned longitudi-

embryo. In the nally. B, Apple seed Cotton seed sectioned longitudinally.

1,1 i e, embryo. B much more

both embryo \ *, A.

J enlarged than A.

and seed coat

are valuable structures. The embryo is rich in oil from which many useful products are made, while the hairs of the seed coat are the Cotton fibers of commerce. (Fig. 64-) The seeds of Clover and Alfalfa are important because the plants which bear them FIG. 64. — Section through increase the soil fertility and are valu- a Cotton seed showing the able for hay and forage, embryo with its much folded Seeds of the Buckwheat and Tomato cotyledons, and the seed coat — , ~ , - ,,.

with the seed hairs. Enlarged Type. — Some common seeds of this about twice. type are those of the Buckwheat, Beet,

Tomato, Potato, Tobacco, Red Pep- per, Coffee, Flax, and Castor-oil plant. The type is common to a number of families, which contain some useful plants and many weeds, such as Nightshades, Spurges, Morning Glories, Bind- weeds, Dodders, Milkweeds, Docks, Smartweeds, and Corn Cockle.

60 . SEEDS AND FRUITS

In structure these seeds differ from those of the Bean type in that they have three distinct parts, an embryo, endosperm, and seed coat, but in number of cotyledons, which is two, the two types are identical. Since endosperm is present, the seeds of this type are known as albuminous seeds. (Fig. 65.) Although some endosperm is always present, sometimes, however, much of it is absorbed by the embryo during the development of the seed, and in this case the cotyledons, which are comparatively

free from stored food in many of these seeds, as- sume some importance as yfe^:S?S$^v storage organs, though not so much as in the Bean tyPe- I*1 tne Buckwheat family, represented by Buckwheat, Rhubarb,

A /} Docks, and Smartweeds,

FIG. 65. — A, section through a Potato and also in some plants seed, c, embryo; e, endosperm; t, testa, of the Goosefoot family, B, section through an achene of Buckwheat. the hardened ovary wall, em embryo; e endosperm; o, ovary wall which when mature re_ and testa. Enlarged.

sembles a seed coat, per- sists as an outer covering over the seed, thus forming with the seed a fruit-like structure known as an achene, a term which is applied to many hard, usually one-seeded fruits, that do not dehisce or, in other words, that do not open to allow the seed to escape.

In external characters, seeds of this type present various differ- ences by means of which one can usually identify the family and often the species to which the seed belongs. Those most com- mon in our region range in size from the smallest of the Dodder seeds, which are almost dust fine, to the size of the Castor Bean. The shape, which in many cases is the chief character by which the family and often the species to which the seed belongs is identified, may be globular, oval, flat, or angled. Such colors as red, yellow, brown, and black are common and serve along with shape and size as a means of identifying different seeds. Sometimes the seed coat is much roughened, as in the Cockle, and in some cases, as in the Milkweeds, the seed coat develops hair-like appendages.

In case of Flax, Buckwheat, Coffee, and the Castor-oil plant,

GRASS TYPE OF SEEDS 61

the seeds themselves are valuable on account of the oil, protein, starch, or alkaloid-like substances which they contain. From the endosperm and embryo of the Flax seed, linseed oil, the chief sol- vent for paints, is obtained. After the oil is pressed out of the flax seed, there remains the cake, which has considerable value as a feed for stock. The Castor Bean yields castor-oil which is much used as a medicine and sometimes as a lubricant and illu- minant. Buckwheat, which contains much starch and some fat and protein, is much used for food when ground into flour. Often, as in case of the Tomato, Potato, Beet, and Tobacco, the value of the plants depends upon the fruit, tubers, roots, or leaves, and not upon the seed, which in these cases has no value except for growing new plants. Of the weed seeds of this type, some com- monly occur as impurities among the seeds of Clover, Alfalfa, Flax, and the small grains and, when present in considerable quan- tities, they either lower the price or prevent the sale of these agri- cultural seeds, thus bringing loss to the farmer. In case of Cow Cockle and Corn Cockle, the seeds, which are frequently found among the small grains, are poisonous and when ground with Wheat make the flour unwholesome and when fed with grain to stock often cause injury. Other weed seeds of this type, as those of Dodder, Morning Glories, Black Bindweed, Sheep Sorrel, and others are objectionable because the plants themselves hinder the cultivation and growth of useful plants. Sometimes, as in case of the Black Nightshade and Jimson Weed, the plants are poisonous.

Grass Type of Seeds. — As the name suggests, these are the seeds of the Grass family, the family to which Corn, Wheat, Oats, Rye, Barley, and Rice belong and hence the family most depended upon for food. Many of the Grass seeds, as in case of Timothy, Red Top, Blue Grass, etc., though not used for food, are valuable because the plants themselves are useful for pasture and hay. Some of the Grasses, however, are regarded as weeds and their seeds are often troublesome impurities among agricultural seeds.

As previously noted, in structure the seeds of the Grass type are not true seeds. Besides a seed, they contain the ovary wall, called the pericarp, which remains about the seed as a closely fitting jacket. They are one-seeded ovaries and hence struc- turally they are fruits rather than seeds. Although popularly known as a seed, this fruit-like structure of the Grasses is scien-

62 SEEDS AND FRUITS

tifically called a cariopsis, a term which refers to its nut-like char- acter.

The seed itself contains three distinct parts, embryo, endo- sperm, and seed coat. The seed coat, however, since it is covered by the ovary wall which performs the protective function of an ordinary seed coat, is poorly developed and so closely joined with the ovary wall that it appears to be a part of its structure. In containing three distinct parts, embryo, endosperm, and seed coat, it is seen that the seeds of the Grass type are identical with those of the Flax and Buckwheat type: but in possessing only one cotyledon instead of two, they are clearly distinguished from both of the other types.

In external features, the seeds of the Grasses present many variations, though probably not so many as occur among some other types of seeds. Most of them are small, but various sizes, ranging from that of a Timothy seed and even smaller up to that of a Corn kernel occur. In most cases they are elongated, and have a groove on one side. In most varieties of Oats and Barley, and in many of the Grasses having very small seeds, the cariopsis remains enveloped by the palea and flowering glume, in which case the entire structure may have the appearance of a seed, especially when the barbed awns and other structures devel- oped by the flowering glume function in dissemination. The seeds of most Grasses are white, gray, yellow or brown, but in Corn such colors as red, blue, purple, and black often occur.

The seeds of those Grasses known as the grains are our chief source of food. Although all of the grains contain practically the same food elements, they differ in the proportion of the differ- ent elements and consequently are fitted for different uses. Even within a seed, various structures differ so much in composition that they are adapted to special uses as is well shown in the milling of Wheat. Likewise in case of Corn, the oil and protein content are so closely related to structure that one can judge the relative proportion of these substances by observing the relative sizes of certain structures of the kernel.

Corn Kernel. — A study of a section through a kernel of corn, as shown in Figure 66, will give a notion of the general structure of the Grass type of seeds. Notice that within the covering (a) there are two distinct regions, that to the right and below being the embryo, and that to the left and above being the endosperm.

CORN KERNEL

63

c°t

The location of the embryo at one side of the endosperm, instead of being centrally located and surrounded by the endosperm, is a peculiar feature of the Grass type of seeds.

The embryo consists of two main parts : the large scutellum or cotyledon (cot) which lies in con- tact with the endosperm, and the embryonic axis which upon germi- nation produces the stem at its upper and roots at its lower end. The axis is attached along its central region to the cotyledon, which supplies it food during growth. At the upper end of the axis is the plumule, a small bud- like structure consisting of a grow- ing point (gr) and some small leaves (I). The plumule is en- closed in a sheath (ct) called col-

eoptile. Between the plumule

, , , , £ , -f FIG. 66. — Section through a ker-

and the attachment of the coty- nel of Corn. cot) cotyledon; ej)) epi.

ledon is a short stem (st), which thelial layer of cotyledon; ct, coleop- with the plumule is often called tile; gr, growing point of plumule; epicotyl (the portion above the z> young leaves; st, epicotyl; r, radi-

cotyledon). The portion of the cle; rc' root caP> cr> coleorhiza; ".

. , . , ,, ,11 • . soft endosperm; h, hard endosperm;

axis below the cotyledon consists 0> covering called pericarp Much

chiefly of the radicle (r), the struc- enlarged.

ture which develops the first root.

The radicle bears at its tip the root cap (re) and is enclosed by

the coleorhiza (cr).

The hypocotyl, which is all or only a part of the axis between the plumule and radicle (a point in dispute among botanists), is the portion of the axis developing least when the embryo resumes growth. In the Grasses there is very little elongation of the hypo- cotyl and, consequently, the establishment of the young plant in the soil and light depends mainly upon the growth of the radicle and plumule. The fact that the hypocotyl remains small while the radicle, since it forms the first root, becomes a prominent structure, accounts for the general application of the term rad- icle to all of the lower portion of the axis, and the rare use of the term hypocotyl in connection with grass embryos.

64 SEEDS AND FRUITS

Concerning the cotyledon of the Grass embryo, there is some dispute. Some morphologists regard the scutellum as the coty- ledon, while others think that the cotyledon includes both the scu- tellum and coleoptile. Although the cotyledon may include other structures, the scutellum, in absorbing and supplying food to the growing parts of the embryo, performs the function of a cotyledon. The scutellum is a boat-shaped structure with its keel-like portion imbedded in the endosperm. Its broad side bearing the axis of the embryo is visible through the testa and ovary wall. The keel-like portion is covered with specialized cells formed into a layer called the epithelium. The epithelium secretes soluble substances called enzymes, which after diffusing to the endosperm change the foods stored there into soluble forms, which are then absorbed by the cotyledon and carried to the plumule and radicle where they are used for growth.

The principal food substances, stored in the endosperm are starch, fat, and protein. Although occur- ring together in most parts of the A B endosperm, each substance is

FIG. 67. — Kernels of Corn with present in a greater proportion high and with low percentages of in gome iong than in otherg> protein. A, kernel with high per- ,-,, „ , ., , , ,.

centage of protein. B, kernel with The Ce"S arOUnd the b°rder °f low percentage of protein, a, horny *he endosperm and forming the endosperm; b, white starchy endo- aleuron layer are especially rich in sperm ;e, embryo. After Bulletin 87, protein, which is present in the University of Illinois Agricultural form of granules and go abundant Experiment Station. ,1^1 n i

that the cells appear as dense

granular masses. The remaining endosperm, which is especially rich in starch, consists of two regions. The outer region (more deeply shaded) is the horny endosperm (h) and contains much protein in addition to starch. The inner region (n) (with lighter shading) is the starchy endosperm, which is not only much softer and more granular than the horny endosperm but also contains less protein. The richness1 of the kernel in protein depends so much upon the amount of horny endosperm that by cutting across a kernel as shown in Figure 67, one may judge the richness of the kernel in protein by observing the relative amount of the two kinds of the endosperm. Likewise, since the embryo of the ker-

1 See Bulletins 44) 82, and 87, University of Illinois Agricultural Experiment Station.

tl ' •

GRAIN OF WHEAT

65

nel contains most of the oil, the oil content depends largely upon the size of the embryo. (Fig. 68.) Sometimes, however, much of the starch of the endosperm is replaced by sugar, as in case of Sweet Corn, which is much used as a -vegetable on account of its soft sweet endosperm.

Grain of Wheat. — In structure, a grain of Wheat is similar to a kernel of Corn. In the section through a Wheat grain, shown in Figure 69, though the parts are not labelled, they can be deter- mined by referring to the section of the Corn kernel shown in

FIG. 68. — Kernels of Corn with high and with low per- centage of oil. A, kernel with large embryo and hence rich in oil. B, kernel with small embryo and low percentage of oil. C and D, face views of two kernels differing in size of embryos and therefore in oil content, e, embryo. After Bulletin 87, University of Illi- nois Agricultural Experiment Station.

FIG. 69. — Lengthwise section through a Wheat kernel. The embryo is to be compared with the embryo of the Corn kernel (Fig. 66} and parts labelled.

Figure 66. In milling1 a grain of Wheat, a number of special products are obtained. The woody pericarp and seed coat with the aleuron layer and some of the outermost starch cells constitute the bran. When bran is finely ground, it is known as shorts. Middlings differ from shorts only in containing a larger percentage of starchy endosperm. In making the best grades of flour, only the starchy endosperm is used and the quality of the

1 On Bread. Bulletin 4, Ohio Agricultural College. Bread and Bread Making. Farmers' Bulletin 389, U. S. Dept. of Agriculture.

66

SEEDS AND FRUITS

flour depends much upon the amount and quality of protein (gluten) which the endosperm contains. When there is a good quality of gluten present, the flour is characterized as being strong and is the kind which.bakers prefer. Graham flour is the entire grain finely ground. In making entire wheat flour, after the grain is finely ground, some of the bran is removed. The em- bryo, which constitutes about eight per cent of the grain, contains much fat and oil, and, if the embryo is ground up with the flour, the oil is apt to become rancid and impair the flavor of the flour. For this reason, the embryo is removed in making high grade

flour and sold with the middlings or used in making breakfast foods. (Fig. 70.)

Oat Kernel. — In general form and structure the Oat kernel is similar to the grain of Wheat, ex- cepting that it is more elongated and the ovary wall is hairy. The kernel usually remains en- closed in the lemma and palea, and the quality of Oats depends

upon the proportion of hull to FIG. 70. — Section through the , , ^i -, , .

- TTrL kernel. I he endosperm contains

outer portion of a Wheat gram, w,

ovary wall, often called pericarp; t, Protein, starch, and fat and is a testa or seed coat; a, aleuron layer valuable food for both man and with cells filled with grains of protein; live stock.

Comparison of Seed Types. —

The three types of seeds differ fundamentally in the number of cotyledons and in the location of the stored food. The difference in the number of cotyledons is probably the more important one because all Flowering Plants have been divided into two classes on the basis of whether or not one or two cotyledons are present. Plants with seeds having but one cotyledon are called Monocotyledons (one cotyledon). Not only the grains and all other Grasses, but also Palms, Lilies, Asparagus, Onions, and many other plants are Monocotyledons. Plants with seeds having two cotyledons, as in case of the Bean, and Buckwheat type, are called Dicotyledons (two cotyledons). Besides Beans and Buckwheat, many other common plants,

g, starchy endosperm with cells large and filled mainly with starch grains. Some protein grains are present in the starch cells, n, nucleus.

RESTING PERIOD

67

such as Peas, Clover, Alfalfa, Tomatoes, Melons, Cotton, Fruit trees, and many forest trees are Dicotyledons. Each of these classes includes a large number of important cultivated plants as well as many that are regarded as weeds.

Since the classification into Monocotyledons and Dicotyledons applies only to the Flowering Plants, such plants as the Larch, Pine, Spruce, Fir, Hemlock, which belong to the Gymnosperms where there are no true flowers, are omitted in this classification. The seeds of a number of the Gymnosperms commonly have more than two and those of the Pine and Cypress commonly have many cotyledons. (Fig. 71.) They are polycotyledonous seeds and the plants may be described as Poly cotyledons.

The difference in the location of the stored food in seeds serves in distinguishing them but does not affect their function or commercial value. In all types of seeds, the endosperm must be absorbed by the cotyledons before it is available for the growth of the embryo. This absorption occurs before germination in the ex- albuminous seeds but during germination in albu- minous seeds. Among Monocotyledons albumi- pine seed nous seeds prevail, while both types are about tioned lensthwise equally common among Dicotyledons. ° ? °w p° ^° '

Resting Period, Vitality, and Longevity of Seeds !?ed germimf "g>

the many cotyle-

The function of a seed as the plant's organ of dons becoming dissemination depends upon a number of physi- free from the seed

a

FIG.

71. — a, see-

ological features, of which the chief one is the

c°? ' , c

ability of the seed to remain alive with the ordi- nary life processes so slowed that they seem not to be taking place at all.

Resting Period. — The resting condition is a valuable physio- logical feature to the seed, because in this condition the embryo can endure cold, heat, dryness, intense light, and various other conditions to which the seed is exposed during dissemination. How the resting condition is brought about and how it is main- tained, often many years, are not thoroughly understood. The resting condition is associated with dryness, a condition obtained

68 SEEDS AND FRUITS

in the seed by allowing the greater part of the water to escape during the process of maturing. Since the life processes depend upon water for dissolving and transporting the necessary sub- stances, they are naturally slowed down when water is with- drawn and apparently without injury even when so checked that no action can be detected by ordinary laboratory methods. Some investigators have maintained that these life processes actually stop, but the evidence sustains the view that these processes never stop so long as the seed remains capable of ger- minating. There are various factors involved in maintaining the rest period, but chiefly they have to do with keeping water and oxygen from the embryo.

The ability of seeds to endure extreme conditions while in the resting stage is well shown in the case of temperature. In liquid air, seeds of Alfalfa, Mustard, and Wheat have been kept at a temperature of —250° C. for three days and afterwards success- fully germinated, though their embryos when active are quickly killed by a temperature a little below freezing. The ability of dry seeds to endure heat is also surprising. Some in the resting stage, if kept dry, can endure a temperature of 100° C., the tem- perature of boiling water, without having their vitality impaired, while their embryos, if active, would perish at 60° C.

The length of the resting period varies much for different kinds of seeds and for seeds of the same kind. In a sample of Clover seed, for example, many of the seeds may germinate in two or three days, and s"ome may not germinate for a month or a year. Although the seeds of some wild plants will germinate as soon as mature, if given favorable conditions of moisture and warmth, most of them, however, have a rest period which extends over days, weeks, months, or even years, and often saves the young plants from getting started at a time when they would soon be caught by unfavorable conditions. Excepting some seeds like those of the Clovers and Alfalfa, the seeds of cultivated plants will usually germinate about as soon as mature. Although a desirable feature, it sometimes results in loss, in that Corn, Wheat, Oats, and other crops germinate in the field if the weather following harvest is warm and wet. The resting period, which is retained by Wild Oats and some other wild plants kindred to cultivated ones, has been lost from our cultivated plants through many years of selection.

VITALITY AND VIGOR OF SEEDS 69

In preventing the absorption of water and oxygen, which are the elements upon which germination in most cases depends, the seed coat and other protective structures are important factors.

Seed coats that prevent the escape of water and thus protect the embryo against excessive drying also prevent the entrance of water, and, if the seed coat is too impervious to water and air, the germination of the seed is delayed. Seeds which have very hard coats, unless they are treated artificially, must be exposed to the weather until the seed coat is decayed sufficiently to allow the entrance of water and air to the embryo before germination can take place. In a sample of Red Clover, Sweet Clover, and Alfalfa seed, often there are many seeds, known as hard seeds, with coats so hard that germination is delayed or prevented. When sown, they either lie in the ground too long before germination or do not germinate at all. By scratching or pricking their seed coats, so that water and air can enter more readily, they germinate more promptly. Experiments1 have shown that Clover seed which has been thrashed through a huller where it is scratched by the spikes germinates much better than seed hulled by hand. This principle is so well recognized that machines especially devised for scratching or pricking the coats of Clover seed have been in- vented. The opening of the seed coats of the Sweet Pea and Canna with a file and Peach pits with a hammer are other in- stances in which the rest period is broken by artificial means.

In some cases, as in the Hawthorne, delayed germination de- pends upon the embryo, which must undergo a process known as " after-ripening " in which acids, enzymes, or other essential sub- stances are formed. In some weed seeds, delayed germination has been found to depend upon the toughness of the seed coat, which allows water and air to enter, but is so resistant to pressure that it will not allow the embryo to expand until its resistance is weakened by decay.

Vitality and Vigor of Seeds. — Seeds are worthless for planting unless they have life, or vitality. Not only the vitality, but also the amount of life or vigor the seed has is an important feature. If the embryo of a seed is dead, the seed will not germinate. If the embryo is lacking in energy, though it may germinate, the plant which it produces will be weak. Only seeds with vigorous embryos are fit for planting.

1 See, Bulletin 177, Vermont Agricultural Experiment Station.

70

SEEDS AND FRUITS

The vitality and vigor of seeds depend upon the following factors: (1) the vigor of the plant which produced the seeds; (2) external conditions which affect seeds during their development; (3) maturity of seeds; (4) weight and size of seeds; (5) methods of storing; and (6) age of seeds.

The seeds of vigorous plants are preferable to those of weak plants, for the sperms and eggs of vigorous plants are likely to be more vigorous than those of weak plants, and, therefore, more capable of producing vigorous embryos. Furthermore, seeds of vigorous plants may have more stored food for the embryo to feed upon during germination and the seedling stage. Plants having a stunted growth, due to drought, lack of food, or attacks of enemies, are likely to produce small and often shriveled seeds which are lacking in stored food and usually have weak embryos.

Seeds are often injured by frosts occurring while the seeds are immature and full of water. The embryos of Corn and other seeds are sometimes killed by early frosts. Even seeds which have reached maturity cannot endure hard freezing unless they are dry. For this reason most seeds should be collected from the field before they have been exposed to a hard freeze.

Abnormal seeds have a low vitality or will not germinate at all. Kernels of Corn produced on the tassel usually give a low per- centage of germination. Sometimes, as in case of Sweet Clover and Alfalfa, when the conditions are unfavorable, seeds are pro- duced with imperfect embryos which are not capable of developing plants. There are some plants in which seeds sometimes develop without embryos and of course will not germinate at all. This sometimes occurs in the Apple and Pear. When seeds are muti- lated their vitality is usually impaired. Larbaletrier asserts that 15 per cent of the Wheat crop in France is injured by the thresh- ing machine. He cut the kernels with a knife so as to represent the injury from the machine and compared their germinative power with that of sound kernels, obtaining a much lower per- centage of germination as the results given in the table below

Sound kernels, per cent of germination.	Cut kernels, per cent of germination.
68 74 99	34 3 38

LONGEVITY 71

show. Sturtevant mutilated the kernels of a Flint Corn and the seeds of Beans and found the percentage of germination much reduced in each case.

Seeds collected while immature usually show a low percentage of germination and their embryos grow slowly. In the case of Rye, seeds have been harvested at different stages of their devel- opment and, after similar treatment in respect to drying and storage, the percentage of germination and vigor of embryos de- termined. In the milk stage five per cent germinated, while in the dry ripe stage eighty-four per cent germinated. The embryos of the dry ripe seeds were much more vigorous in growth than those of the immature seeds. Tomato seeds, while still green and not more than two-thirds the weight of mature seeds, may be germinated, if properly cured, but the plants produced are likely to be weak. The germination of unripe seeds has been given considerable attention by Sturtevant, Arthur, and Golf.1

Experiments 2 with seeds of the Radish, Sweet Pea, Cane, Rye, Oats, and Cotton have shown that better stands in the field and more vigorous and better yielding plants are secured by using only the heavier seeds.

The vitality and vigor of seeds depend very much upon the methods of storing. Seeds are more easily killed by extremes of temperature when wet. Seeds stored where there is considerable moisture may start to germinate, and then die. Seeds, massed together before they are well dried, become moist and often so warm that the embryos are injured. On the other hand, when stored in rooms where the air is warm and extremely dry, seeds may lose moisture so rapidly that the embryos are killed. A storage room should be cool but above freezing, and dry, although not excessively dry. Until the seeds are well dried, they should not be massed together, but so arranged that the air can circulate about them. Thus methods of storing seed Corn and other seeds must reckon with a number of factors which affect the vitality and vigor of seeds under storage conditions.

Longevity. — The vitality and vigor of seeds depend much upon their age. Seeds in excellent condition and stored by the best methods finally lose their vitality, due to the coagulation of their protoplasms, too much drying, or some other factor not under-

1 American Naturalist, pp. 806 and 904. 1895.

2 Farmers' Bulletin 676, U. S. Dept. of Agriculture.

72 SEEDS AND FRUITS

stood. Some seeds may retain their vitality for centuries, but most seeds lose it in a few years. The length of time during which seeds retain their vitality is called their longevity. Most agri- cultural seeds can be stored two or three years without much loss of vitality, and some, when stored a much longer period, may contain a large number of live seeds. One investigator found that 50 per cent of samples of Red Clover seeds germinated after being stored in bottles for 12 years; and in samples of the seeds of Pigweed, Sheep Sorrel, Black Mustard, and Pepper Grass, stored in the same way, a large percentage germinated after a storage of 25 years. In samples of White Sweet Clover seeds, which have well modified seed coats, 18 per cent have germinated after a storage of 50 years. There is good evidence that some of the leguminous seeds may retain their vitality for more than a century. Many of the weed seeds when buried in the soil can retain their vitality for many years and then germinate when conditions become favorable.

The longevity of seeds depends so much upon the conditions under which the seeds were grown, maturity when collected, and methods of storing, that statements as to how old any kind of seeds may be and still be safe for planting are not reliable. Old seeds are often preferable to new ones grown under unfavorable conditions. Seeds from poorly developed plants, although sim- ilar in appearance to those produced under favorable conditions and giving a high percentage of germination soon after harvest, decline rapidly in vitality, often being worthless at the next plant- ing season. For example, Cabbage seeds eight years old may germinate 70 or 80 per cent, while some only three years of age but grown in an unfavorable year may germinate less than 40 per cent. Seeds collected green may germinate well after proper curing but they have a short longevity.

The longevity of seeds depends probably more upon dryness than any other factor. For this reason the place of storage should be dry and the seeds should be cured before they are stored by placing them in a dry airy place. Experiments show that Corn collected soon after maturity and properly cured and stored gives a much higher percentage of germination the next season than Corn allowed to stand in the shock, or taken from the crib. Comparative L germinative tests of seeds stored in different 1 Bulletin 58, Bureau of Plant Industry, U. S. Dept. of Agriculture.

LONGEVITY

73

parts of the United States have shown that seeds do not live as long in the warm moist air of the Southern states as they do in the cool dry air of the Northern states.

In the following table compiled from various sources is given the time "beyond which it is not advisable to use the seeds men- tioned unless the contrary is shown by germinative tests.

Corn

Wheat

Oats

Barley .

Rye

Buckwheat

Beans (common) 4 to 5

Peas 4 to 5

Clovers 2 to 3

Alfalfa 3 to 4

Onion 1

Years. 2 2 2 2 2 2

Years.

Mustard 3 to 4

Cabbage 3 to 4

Turnips 3 to 4

Swede 3 to 4

Pumpkin 5

Melon (musk) 5

Melon (water) 5

Squash 3

Tomato 6

Timothy 1 to 2

Celery 1

In some cases perfect seeds well stored may have more than double the longevity given in the above table. Thus Sturtevant obtained 100 per cent germination of various varieties of Corn after being stored 5 years. Tomato seeds 14 years old have been known to give a high percentage of germination. On the other hand, using the same seeds as an example, both Corn and Tomato seeds are sometimes unfit for use when only 1 year of age. These varying results em- phasize the importance of testing the germinative power of seeds before use.

The variation in the longevity of the seeds of a given lot is obvious when the percentages of germination for different periods of storage are compared. The decrease in the percentage of germination as the length of the storage period in- creases shows that some seeds die early magnifier from above, and others later until finally all are dead.

In the following table are given the results of an experiment to determine the rate at which vitality is lost as indicated by the percentage of germination obtained in each of the 6 years of storage.

FIG. 72. — A cheap mag- nifier well adapted for use in analyzing seeds. The magnifier is set over the seeds, leaving the hands free to separate the seeds as one looks through the

74

SEEDS AND FRUITS

PER CENT OF GERMINATION FOR EACH OF 6 YEARS OF STORAGE

Seed.	lyr.	2yr.	3yr.	4yr.	5 yr.	6yr.
Wheat	80	82.3	77.3	37	15	6
Oats	90 2	93	78.2	67	54	29.5
Barley	97	91	78.5	36	19.5	7.5
Peas	94	95	88	64	64	6
Flax	81	82	75	49	26	24

Purity and the Analysis of Seeds

The impurities of seeds consist of seeds of other species and of dirt, such as soil particles, chaff, hulls, and other plant fragments. In sowing impure seeds one can not estimate the amount of desirable seeds sown unless the percentage of impurities is pre- viously determined so that allowance can be made. Besides one is likely to sow the seeds of undesirable plants, which choke the crop and cause much trouble and expense in eradicating them. A small per cent of weed seeds is often a serious matter. For example, in sowing Grass seeds which contain only 1 per cent of weed seeds there is the possibility of 20 or more weeds to the square yard. Nobbe found enough weed seeds in a certain sample of Timothy seeds, if sown at the ordinary rate, to supply 24 weeds to every square foot of land. Furthermore, in purchas- ing impure seeds, unless a deduction from the price is made for the impurities, one pays more than he should for the desirable seeds obtained.

More impurities occur among the smaller agricultural seeds, as Grass, Clover, and Alfalfa seeds, than among the grains, al- though a few very bad weed seeds, such as those of Quack Grass (Agropyron repens), Cow Cockle (Saponaria Vaccaria), Corn Cockle (Lychnis Githago), and English Charlock (Brassica Sinapistrum) , are common among the small grains.

Seed Analysis. — A bag of seeds may be analyzed for two reasons: (1) to determine the percentage of the desirable seeds contained or to determine the percentage of impurities regardless of their kinds; and (2) to determine the kinds of impurities and the percentage of each present. In either case the determination is based upon the analysis of only a small sample, which is usually prepared by mixing well a handful or more of seeds taken from

SEED ANALYSIS

75

different parts of the bag or container, usually from the top, middle, and bottom. From the sample from 2 to 5 grams are weighed out, and the impurities and desirable seeds are then sepa- rated, usually by means of a lens like the one in Figure 72. By

JN'f

"a'w ^^ *\jk

UCanad^cx 15WAA ^

FIG. 73. — Some weed seeds and fruits commonly found among Red Clover seeds. Enlarged and about natural size. From Farmers' Bulletin 455, U. S. Dept. of Agriculture.

dividing the weight of the desirable seeds and the weight of the impurities by the number of grams analyzed, the percentage of each is obtained. Thus, if 5 grams are analyzed and the weight

of the desirable seeds found is 4.8 grams, then -^- = 96 per cent,

o

which is the percentage of purity. In determining the kinds of

76

SEEDS AND FRUITS

impurities and their percentages it is not enough to separate the impurities and desirable seeds, but the kinds of impurities must be identified, separated, and the weight necessary for finding the percentage of each must be separately determined. In this kind

1 Jl/fe/fa 2 Yd/owfafoil 3 "* Burcfonr

WM

<!> $

7 'Ufa 'mustard 8 Cttr/e</</oc/c

9 ftussiarr thisf/e

10 Lamb's quarters

FIG. 74. — Some weed seeds commonly found among Alfalfa seeds. En- larged and natural size. Adapted from Farmers' Bulletin 495, U. S. Dept. of Agriculture.

of analysis, the operator, unless he is well acquainted with the various kinds of seeds, should have at hand for comparison samples or figures of the seeds of weeds and other plants likely to occur among the seeds which are being analyzed. Samples are better, but figures as shown in Figures 73 and 74 may serve quite well.

TOMATO OR BERRY TYPE

77

Nature and Types of Fruits of Flowering Plants

A fruit is difficult to define because not all fruits involve the same structures in their formation. Some fruits are only much enlarged ovaries; but there are others which involve other struc-

-5

-a

FIG. 75. — A, cross section of a Tomato. B, cross section of an Orange. w, ovary wall; p, placentas; s, seeds; a, partition walls; I, locules.

tures closely related to the ovary. Since fruits involve a number of structures in their formation, it will be best to study some types and then formulate a definition.

Tomato or Berry Type. — The fruit of the Tomato consists of the ovary which has enlarged and become fleshy and juicy. The most edible portion consists of the fleshy enlargements which develop from the inner angle of the locules and almost fill them. These enlargements bear the seeds and hence are the placentas much enlarged. Also the citrus fruits, such as Oranges, Lemons, etc., are of the berry type. However, they have no fleshy placentas. The seeds are attached to the small central core, and the juicy tissues developing from other parts of the ovary and filling the locules constitute the flesh of these fruits. The fleshy and juicy features are characteristics of the berry; and a berry is often defined as a fleshened juicy ovary. (Fig. 75.)

S

FIG. 76. — Lengthwise section through a Plum, s, seed; p, wall of pit; /, fleshy portion of ovary.

78

SEEDS AND FRUITS

Plum or Stone Type. — The Plum, Peach, Cherry, and Apri- cot, commonly called drupes, are fleshy ovaries, but differ from

-r

s B

FIG. 77. — Section through flower and fruit of the Apple. A, section through the flower, a, receptacle; b, ovaries; d, ovules; t, floral organs, calyx, corolla, stamens, styles and stigmas. B, section through the fruit, a, receptacle; c, core; s, seeds; r, remains of floral parts; I, the flesh around the core, bounded on the outside by the conductive vessels, indicated by the lines. The inner portion of this band of flesh is the outer portion of the ovaries, the remainder of it being the inner portion of the receptacle.

the berry type in that the portion of the ovary immediately sur- rounding the locule hardens into the stone or pit. In Figure 76, point out the seed, the pit, and the fleshy portion of the ovary.

Apple or Pome Type. — The Apple, Pear, and Quince are examples of pome fruits, and their structure can best be un- derstood by studying Figure 77. . The receptacle of the flower is not flat as it is in many flowers, but is hollow or urn- shaped; and the five ovaries are located in the hollow of the receptacle and are grown fast to its sides. The calyx, petals, and stamens are located on the rim of the receptacle and thus above the ova- ries. As the fruit develops, the receptacle surrounding the ovaries thickens and forms the greater part of the fruit, while the ovaries form the portion known as the core.

FIG. 78. — Cross section of a Cucumber, r, rind consisting of receptacle and ovary wall closely joined; I, locules; p, pla- centas; s, seeds.

BLACKBERRY TYPE

79

Melon or Pepo Type. — In the Melons, Cucumbers, Pump- kins, and Squashes, which illustrate well the pepo type, the ovaries are inclosed in the receptacle, and with the receptacle to

! FIG. 79. — Flower and fruit of Strawberry. A, section through flower, showing the fleshy receptacle (r) and the many pistils (p) on its surface. B, fruit consisting of enlarged receptacle (r), bearing the small hard ovaries (o).

which they are closely joined form the rind. (Fig. 78.) The placentas are more or less fleshy and in case of the Watermelon, where they form large juicy lobes, they constitute the bulk of the edible portion. In most cases, however, as Muskmelons and Pumpkins illustrate, the placentas break loose from the ovary wall and are removed with the seeds. In what way does the Melon resemble the Apple in structure? How does it differ from the Apple?

Strawberry Type. — In the Strawberry the ovaries develop into hard one-seeded fruits (akenes) which appear as small hard bodies over the surface of the much flesh- ened receptacle. (Fig. 79.) In the Straw- berry, although the ovaries are included when the fruit is used, the edible portion is the receptacle.

Blackberry Type. — In this type the

u £ f, £j_ FIG. 80. — Fruit of the

ovaries develop as small stone fruits, often m , u

* ' Blackberry, r, recepta-

called drupelets (miniature drupes), and cle; /, fleshened ovaries, with the fleshened receptacle form the

fruit. (Fig. 80.) Very similar to the Blackberry is the Rasp- berry, in which the drupelets collectively separate from the re- ceptacle and thus alone form the fruit.

80

SEEDS AND FRUITS

In developing from a single flower but involving a number of pistils, the fruits of the Strawberry and Blackberry are similar and are classed as aggregate fruits.

Pineapple Type. — In the formation of the Pineapple a num- ber of flowers are involved, each of which consists of a small pistil surrounded by large scales and is borne in the axil of a modi- fied leaf. Each ovary with its scales and modified leaf becomes fleshy to form a single fruit. The entire fruit of the Pineapple consists of a number of these single fruits closely packed together on an axis which forms the core of the Pineapple. Since a number of flowers are in- volved, fruits of this type are known as multiple fruits. (Fig. 81.)

Nut Type. — In the nut type of fruit, the ovary is hard and is generally partly or entirely covered by a husk formed by the perianth or by bracts which grow up from the receptacle. (Fig. 82.) Notice the develop- ment of the Acorn shown in Figure 83.

Some Other Familiar Types of Fruits. — In many small fruits the ovaries become dry and often hard as the fruit matures. They are the kind which when small and one-seeded are often called seeds. It has been mentioned that the akenes of the Buck- wheat and the cariopsis of the Grasses are fruits with hard ovary walls. In the Clovers, Alfalfa, and Beans the ovary wall becomes dry and hard when mature, forming the structure known as the pod or legume. (Fig. 84-) Many of the so-called weed seeds are dry ovaries. In many cases, however, other structures are joined with the hardened ovary in the formation of the fruit. In the Dandelion and many other plants of the Composite type, the

FIG. 81. — Pineapple. After Koch.

DEFINITION OF A FRUIT

81

pappus, consisting of hair-like structures which correspond to the calyx of the ordinary type of flower, remains as a part of the fruit, forming a parachute-like arrangement which enables the

FIG. 82. — Pistillate flower and fruit of a Hickory (Gary a). A and B, ex- terior and interior views of the flower. C, the nut. 6, bracts surrounding the pistil (p)', o, ovary. Flower much enlarged but fruit reduced.

fruit to float in the air. Sometimes, as in the Spanish Needles, the calyx remains on the fruit as spiny appendages. In the case of the Birch, Elm, Ash, and Maple, the fruit known as a samara or key-fruit has wing-like structures which are outgrowths from the ovary wall.

B

FIG. 83. — Flower and fruit of an Oak (Quercus). A, pistillate flower, showing the bracts (6) which surround the ovary. B, section of the flower, showing the ovary (o) and the bracts (6). C, acorn, showing the ovary and cup. s, stigmas. Flower much enlarged but fruit nearly natural size.

Definition of a Fruit. — From an examination of the above types of fruits, it follows that a fruit may consist of: (1) simply the ovary either dry or fleshy; (2) ovary or ovaries and recep-

82

SEEDS AND FRUITS

tacle; (3) ovary with perianth or bracts forming a husk; (4) ovary with calyx forming hairs or spines; and (5) a number of single fruits with the modified leaves and floral axis of the flower group. A fruit may be defined as one or more ripened ovaries either with or without closely related parts.

Dissemination of Seeds and Fruits

Dissemination has to do with the scattering of seeds from the parent plant. Sometimes the seed is transported naked, but often it is transported enclosed in the fruit or with some larger part of the plant.

The necessity for dissemination is ob- vious, for if the seeds of a plant were to FIG. 84. — The dry coiled germinate where formed or on the ground fruits (pods) of Alfalfa directly beneath, the resultant conges- (Medieago sativa). From tion would prevent the normal develop- Farmers' Bulletin 895, U.S. ment of any of the plantg> Green

Dept. of Agriculture. , ^• ^ , , •

plants must have sunlight and air, and

this means that they must have room.

Of course seeds and fruits are not the only means by which plants spread. Many Seed Plants have an additional means in either spreading stems or roots which give rise to new plants as they spread farther and farther from the parent. The Straw- berry depends mainly upon its runners, and the Quack Grass much upon its underground stem as a means of spreading. Pop- lars, some fruit trees, and Canada Thistle are well known to spread by means of sprouts arising from their roots. Most plants which do not have seeds spread by means of spores which in some cases seem to be a more efficient means than seeds are. For example, Wheat Rust, a disease which spreads very rapidly, is spread by spores.

In the dissemination of seeds and fruits, wind, water, and ani- mals are the chief agents. In a few plants there are explosive or spring-like mechanisms which throw the seeds.

Seeds and Fruits Carried by Wind. — The wind is one of the most important agents in the distribution of fruits and seeds. In

SEEDS AND FRUITS CARRIED BY WIND

83

the Thistle, Dandelion, Wild Lettuce, Fireweed, Iron weed, White Weed, Fleabane, and others, the tufts of downy hairs on the small dry fruits in which the seeds are enclosed enable the fruits with the seeds to be lifted and carried many miles by the wind. In the Milkweeds, the seeds bear long hairs which make them easily carried by the wind. In some plants, as in the Curled and Smooth Dock, Ash, Elm, and Maple, the fruits are winged and easily borne away by a passing breeze. The fruits of some of the

a

FIG. 85. — Some fruits and seeds disseminated by the wind, a, fruits of the Basswood (T ilia Americana) and the leaf -like bract which floats in the air and thereby scatters the fruits. 6, samara or winged fruit of a Maple, c, fruit of a Wild Lettuce (Lactuca Floridana). d, winged fruit of an Elm. e, pods of a Milkweed (Asclepias syriaca) allowing the seeds to escape to be scattered by the wind, a, c, and e from Hayden.

Grasses are enclosed in chaff bearing long hairs and are easily blown about. The fruits and seeds of Ragweeds, Velvet-leaf, Docks, Pigweeds, Chickweeds, and some plants of the Grass fam- ily are blown long distances over the surface of snow, ice, or frozen ground. (Fig. 85.)

Some plants break off near the ground after ripening their seeds and are rolled over and over by the wind, dropping their seeds as they go. These are known as the "tumble-weeds" and include the Russian Thistle, Tumbling Mustard, Tumbling Pigweed, Buffalo Bur, Old Witch Grass, and a number of others. (Fig. 86.)

84

SEEDS AND FRUITS

Seeds and Fruits Carried by Water. — Plants, such as the Great Ragweed, Smartweeds, Bindweeds, Willows, Poplars, and Walnuts, which grow along streams, have their seeds and fruits floated away during overflows. Sometimes, when the banks of

FIG. 86. — Plants of the tumble weed (Amaranthus albus} tumbling over the ground and scattering seeds as they go. After Bergen.

streams cave off, plants with ripened seeds fall into the current bodily and are carried for miles down the stream, finally lodging in fields where their seeds grow. The seeds of plants growing on the upland are washed to the lowlands during rains and seed the bottom fields. Some fruits, as in case of the Coconut, are so resistant to salt water that they can be carried long distances by ocean currents.

Seeds and Fruits Carried by Animals. — Birds eat the fruits of some plants for the outer pulp, and the hard seeds pass undi- gested. In this way .the seeds of the Nightshades, Poison Ivy, Poke weed, Blackberry, Pepper Grass, and others are distributed. Even the seeds and fruits of Thistles, Dandelion, Ragweeds, and Knotgrass may be eaten in such large quantities that many pass undigested and start new plants wherever they fall. Birds often carry sprigs of plants to places where the seeds may be eaten without molestation and in this way distribute seeds. (Fig. 87.) Birds that wade in the edge of ponds, lakes, and streams often carry away on their feet and legs mud containing seeds.

SEEDS AND FRUITS CARRIED BY ANIMALS

85

Darwin took 3 tablespoonfuls of mud from beneath the water at the edge of a pond and kept it in his study until the seeds con- tained developed into plants. From this small amount of mud, he obtained 537 plants which represented a number of species. From this it is evident that the rnud, carried on the feet and legs of water birds, may be the means of distributing many seeds.

The fruits and seeds of many plants have spines, or small hooks by which they become attached to passing animals and are carried far and wide. Some familiar ex- amples are the burs of Burdock, Cockle Bur, and Sand Bur, and the hooked and spiny fruits of the Buttercups, Wild Carrot, Beggar's Lice, Tick-trefoils, Beggar-ticks, and Spanish Needles. They catch

• • 1 i T«T/» J- J.\JC. <^J I . J-i V-/J.a.iVJ.

in the wool, manes, and tails of fruit_ From Bulletin stock and in the clothing of man, logical Survey, and are carried from one pasture

to another or from one farm to another. Live stock are impor- tant agents in distributing plants on the farm. The seeds of the Mustards are mucilaginous when wet and, by sticking to the feet of animals or the shoes of man, are carried to new situations. (Fig. 88.)

Many plants owe their distribution to man more than to any other agent. The railways, connecting all of the states and reaching from ocean to ocean where they connect with steamship lines from across the seas, are responsible for the wide distribution of many plants. For example, the seeds of a number of weeds are shipped across the country with grain and other farm seeds, and also in hay, bedding, packing, in shipments of fruit, and in the coats of live stock. They fall from the cars as the train travels, and seed the right-of-way where the plants first appear and then later spread to the surrounding fields. The railways are responsible for the wide distribution of Russian Thistle, Prickly Lettuce, Canada Thistle, and Texas Nettle, which first appear along the railway and later spread to the surrounding

FIG. 87. — A Chickadee carrying Iowa Geo-

86

SEEDS AND FRUITS

farms. Buckhorn, Ox-eye Daisy, and many other weeds are often first found along the railway. Seeds of various kinds are often carried in the packing around nursery stock. Quack Grass Canada Thistle, Ox-eye Daisy, and other weeds are often spread

FIG. 88. — Some spiny weed fruits which catch to the coats of animals, a, cow with tail loaded with weed fruits. 6, fruits of Beggar-ticks (Bidens). c, spiny fruit of Burdock (Arctium Lappa). d, fruit of Comfrey (Symphytum} . e, fruit of another Beggar-tick. Adapted from Bailey and from Hayden.

in this way. Quack Grass is often carried in straw, and may be introduced on a farm by using straw for covering Grapes and Strawberries. Manure hauled from livery stables is a very im- portant means of introducing plants on the farms where the manure is used. In hauling hay along the highways, seeds of various kinds are dropped and from the highways the plants spread to the fields. Those weeds, such as Quack Grass, White Top, Field Sorrel, and others which are common in meadows, are often spread in this way. When the fields are wet, seeds

SEEDS SCATTERED BY EXPLOSIVE MECHANISMS 87

collect on the wagon wheels and are carried to the highways or to other fields. Threshing machines are important agents in scattering seeds, for in their traveling through the country seeds of various kinds are jostled from them and seed the fields and highways.

Man scatters many weeds by sowing unclean seed. Clover seed, Alfalfa seed, Grass seed, Wheat, Oats, etc., are often ob- tained from distant states or even from foreign coun- tries for seeding. Weed seeds are usually present FIG. 89. — The three-valved pod of the in agricultural seeds, and Violet throwing its seeds. Much enlarged, sometimes they are pres- e

ent in large quantities. In tracing weeds, it has been found that many of the most troublesome ones have come from Europe, Asia, or some other foreign country. Man has carried the seeds and fruits of these weeds across the seas, and most of them have

been imported and sown with agri- cultural seeds.

Seeds Scattered by Explosive or Spring-like Mechanisms. — In this kind of dissemination the plant itself is the agent which, either by sudden ruptures due to strains or by explosions due to the swelling of certain tissues, is able to throw the seeds often a considerable distance. In the pods of some plants, as in the Vetches, Witch-hazel, Castor FIG. 90. — The Squirting Cu- Bean, and Field Sorrel, bands of cumber (Ecbalium Elaierium) tisg which ri under tension, squirting its seeds from the pod. . ,, , ,, -,

exert such a strain that the pods

suddenly rupture with so much violence that the seeds are thrown in every direction. In the Violets the carpels, as thdy ripen and dry, press harder and harder upon the seeds, which suddenly

88 SEEDS AND FRUITS

shoot out as a Watermelon seed may shoot out from between one's pressed fingers. (Fig. 89.) In case of the Impatiens called " Touch-me-not " and the " Squirting Cucumber/' tissues within the pod take up water and swell so much that the pod finally explodes and scatters the seeds as shown in Figure 90.

CHAPTER VI

GERMINATION OF SEEDS: SEEDLINGS Nature of Germination and Factors upon which it Depends

Although the resting condition is very essential to the preser- vation of the life of the seed during transportation and while awaiting favorable conditions for germina ion, it must be aban- doned at some time in order that the embryo may develop into the plant, the . production of which is the seed's chief function. By germination of a seed is meant that awakening from the rest- ing condition in which the young plant shows practically no signs of life to a state of active growth. The term germination is used in different ways, being used to designate the beginning growth of such structures as a pollen tube, fertilized egg, and spore, but in each case, however, it refers to the initial growth. Seeds are considered germinated when the radicle and plumule have broken through and project beyond the seed coverings, although germination is not complete until the little plant is able to live independently of the stored food of the seed.

Conditions Necessary for Germination. — The awakening of the seed into active growth depends upon the presence of warmth, moisture, and oxygen. Germination is so dependent upon these three external factors that, if either is lacking though the other two are properly supplied, there will be very little or no germination. Among different seeds, the degree of temperature and the amount of moisture and oxygen required for the best germination vary.

Temperature Requirement. — Seeds vary more in the temper- ature required for germination than in any other factor. Through experience we have learned that among farm and garden seeds there are different temperature requirements for germination, and that the time of season at which different seeds should be planted must be chosen accordingly. Thus Oats, Wheat, and Red Clover seeds, which have a low temperature requirement, can be planted in the early spring or late fall when the weather and soil are

90

GERMINATION OF SEEDS: SEEDLINGS

cool, but if Corn or Melons, which have a high temperature re- quirement, are planted before the weather and ground are warm they will decay and have to be replanted. In considering tempera- ture in relation to germination, three temperatures are usually noted; the minimum, the lowest temperature at which germina- tion will occur; the optimum, the temperature most favorable for germination; and the maximum, or highest temperature per- mitting germination. As the following table shows, these tem- peratures are very different for different seeds, sometimes differ- ing as much as 25° or 30° (Fahrenheit).

GERMINATION TEMPERATURES (FAHRENHEIT)

Kind of seeds.	Minimum.	Optimum.	Maximum.
Oats	Deg. 32-41	Deg. 77- 88	Deg. 88- 99
Wheat, Rye Indian Corn	32-41 41-51	77- 88 99-111	88-108 111-122
Red Clover Peas	32-41 32-41	77- 88 77- 88	99-112 88- 98
Sunflower	41-51	93-111	111-122
Pumpkin	51-61	93-111	111-122
Musk Melon	60-65	88- 99	111-122
Cucumber	60-65	88- 89	111-122

Germination, which proceeds most rapidly at the optimum temperature, decreases in rate as the temperature approaches the minimum or maximum as the following table shows in case of Corn, in which the time required for the radicle to break through, though only 2 days at the optimum temperature, was 10 days in a temperature near the minimum. In the majority of cases, the temperature of the soil in which seeds are planted is somewhat below the optimum and, consequently, if the soil tem-

EFFECT OF TEMPERATURE ON RATE OF GERMINATION

	Germinating Period in Hours.
Temperature ° F.
	Indian Corn.	Red Clover.
42	240	180
55	144	32
75	56	24
87	48	24
102.6	48	24
111.2	80

MOISTURE REQUIREMENT 91

perature is lowered as it often is by heavy rains which fill the soil with water or by days of cool cloudy weather, germination is either very slow or prevented as is well known to every farmer and gardener.

Moisture Requirement. — The amount of moisture required for germination is, in general, that which will completely saturate and soften the seeds. The water absorbed saturates the cell walls and starch grains, and fills the living cells of the embryo and all empty spaces that exist in the seed. Although the amount of water required to saturate different seeds varies, it is always a large per cent, sometimes more than 100 per cent of the dry weight of the seed, as shown in the table below. Reckoning in pounds from the percentages given in the table, 100 Ibs. of Corn after being soaked for germination may weigh 144 Ibs. and 100 Ibs. of White Clover seeds after soaking may weigh 226.7 Ibs.

WATER ABSORBED BY GERMINATING SEEDS

Seeds.	Per cent of water absorbed in germination.
Indian Corn.	44
Wheat	45 5
Buckwheat . .	46 9
Rye .	57 7
Oats White Beans	59.8 92 1
Peas	106 8
Red Clover .	117 5
Sugar Beet	120 5
White Clover .	126 7

Most seeds, though not all, swell as water is absorbed, some- times more than doubling their dry size. In fact, the per cent of increase in volume is often greater than the per cent of water absorbed, as in case of the Pea which may increase in volume 167 per cent while absorbing only enough water to increase its weight about 100 per cent.

If seeds are confined in a space which they fill when dry, their swelling may exert a force of several hundred pounds and often sufficient to break strong containers. This force is sometimes used in opening skulls in anatomical laboratories, in which case the skulls are filled with dry Peas, which after being moistened swell and force the bones apart.

92

GERMINATION OF SEEDS: SEEDLINGS

Oxygen Requirement. — Although seeds are in the optimum temperature and properly supplied with moisture, they will usu- ally not germinate unless oxygen is supplied, as is often demon- strated in the laboratory by the use of some substance to absorb the oxygen in the germinator or by replacing the air in the ger- minator with hydrogen, nitrogen, or some other substance, so that oxygen is excluded. (Fig. 91 .) However, since the air is about one-fifth oxygen, seeds receive enough oxygen to germinate well if only air is supplied, although germination is often hastened

--S- -

FIG. 91. — The two U-shaped tubes, which contain soaked seeds (s) on moist blotting paper at their stoppered ends, are alike except that in B the open end of the tube is in pyrogallate of potash, which absorbs the oxygen from the air in the tube, while in A the open end of the tube is in pure water, in which case the oxygen still remains in the air of the tube. The seeds ger- minate well in A but not in B.

when the amount of oxygen is increased artificially. For exam- ple, in an experiment Wheat, requiring 4 to 5 days to germinate in the air, germinated in 3 days in pure oxygen. There are a few seeds, however, which begin to germinate without oxygen, but they soon die unless oxygen is supplied.

For lack of oxygen seeds germinate poorly when planted in the soil so deeply that not enough air is accessible, or when planted in soils with their pores so full of water that the circulation of the air is prevented.

CHANGES IN THE STORED FOOD 93

Germinative Processes

Seeds need water, oxygen, and warmth in germination because upon these external factors the internal germinative processes depend. For dissolving and transporting foods water is indis- pensable; the occurrence of certain chemical processes depends upon oxygen; and in order for both chemical and physical proc- esses to be suitably active, as previously shown (page 90), warmth is required.

Changes in the Stored Food. — The first of the germinative processes has to do with the digestion and translocation of the stored foods. Whether stored outside of the embryo or in the cotyledons, the stored foods, until brought nearer, are beyond the absorptive reach of the cells of the plumule and radicle where they are most needed. But unless foods are in solution, which is the only form in which they can pass through the walls and proto- plasm of cells, they can not move from one region of a plant to another. Therefore, since starch, fat, and protein, which are the chief storage foods of seeds, are not readily soluble in water, they must be changed to sugar, fatty acids, peptones, or other soluble forms before being transported. However, this digestive process occurs not only in seeds but also in all plant regions where foods are transported, and also in animals it has its likeness in the diges- tive process by which foods are made soluble, so that they can -pass through the walls of the alimentary canal to the blood, which carries them in solution throughout the body. Both the digestion and transportation of the stored foods are quite noticeable during the germination of some large seeds, as in case of Corn in which the endosperm becomes watery and disappears as germination proceeds, or in case of Beans where the cotyledons in which the food is stored gradually shrink as the young plant develops.

The digestive process in plants as well as in animals is per- formed by special substances known as enzymes, which in case of the seed are secretions of the embryo. Enzymes occur in solu- tion, either dissolved in water or in protoplasm, in all parts of the plant where they either initiate or hasten chemical changes. They are exceedingly important substances because upon them the majority of chemical changes in plants depend. They are specific in their action, that is, as a rule, each enzyme acts on only one kind of a substance, and is concerned with only one or two

94 GERMINATION OF SEEDS: SEEDLINGS

chemical changes. Consequently, the kinds of enzymes are almost as numerous in the plant as the kinds of substances to be acted upon. Thus for changing starch into sugar there is the enzyme known as diastase which is especially active in seeds, but common in other plant organs and in animal saliva. An enzyme secreted by the Yeast Plant and called zymase acts on sugar, forming besides alcohol, carbon dioxide which puffs up the dough when Yeast is used in bread-making. This enzyme also occurs in seeds, fruits, and other plant organs. Lipase converts fats into soluble fatty acids, and pepsin changes insoluble proteins into peptones and other soluble forms. Then there are oxidases, en- zymes which oxidize substances as the name suggests, and perox- idases which take oxygen away from compounds, and many other enzymes which play an important role in the chemical activities of the plant. The exact chemical nature of enzymes has never been determined because of the difficulty in separating them from other protoplasmic substances which enter into and thus compli- cate the analysis. Nevertheless, there is much evidence that enzymes are protein-like substances. One striking feature of an enzyme is that it does not enter into the chemical action which it causes, and, therefore, a small quantity of an enzyme can keep a chemical action going until a large quantity of a substance is changed.

Although all living cells, whether in the embryo or elsewhere, produce enzymes, sometimes, however, certain cells have the secretion of enzymes as their special function, as in Corn, Wheat, and other seeds of the Grass type, where the epithelial layer of the scutellum has for its special function the secretion of the diastase and other enzymes which are necessary for converting the endo- sperm into soluble forms.

Transportation of Soluble Foods. — After the foods are made into soluble forms and dissolved in the water present, they pass from one region of the plant to another by the physical processes known as diffusion and osmosis. Diffusion is probably better known among gases where the spread of odors through a house, the fragrance of flowers through gardens, and smoke through the air are everyday illustrations of it. The spread of indigo, ink, or any substance like salt and sugar through the water in which they are dissolving illustrates it. By diffusion substances, whether dissolved in a gas or a liquid, spread farther and farther

-I

ELABORATION OF FOODS INTO PLANT STRUCTURES 95

from the place where they entered the dissolving medium, and thus toward those regions where they are less concentrated. In case a number of substances are in solution at the same time, each diffuses independently of the others. When, for example, sugar, salt, and ink are dissolved in a vessel of water at the same time, each diffuses to all parts of the vessel independently of the others and, consequently, the substances become thoroughly mixed just as the oxygen, nitrogen, carbon dioxide, and other gases of the air by diffusion tend to thoroughly mix. It is apparent then in case of the seed that foods in a concentrated solution in the endo- sperm or cotyledons will diffuse to the radicle and plumule, where the food, by being constantly removed from the 'solution to be built into plant structures, is kept less concentrated.

Osmosis mentioned as another process involved in the trans- portation of foods is also a diffusion, but differs from the ordinary diffusion just described in that it takes place through a membrane which alters the rate of the diffusion of different substances by allowing some to pass through it more readily than others. It is by this kind of diffusion that substances pass into and out of liv- ing cells, in which case the membrane through which the sub- stances must diffuse is the modified border of the protoplasm. Thus, although foods depend much upon ordinary diffusion for transportation when not passing through membranes, in entering or leaving living cells they must also depend upon osmosis, the nature and principles of which are more thoroughly discussed in connection with the cell (Chapter VII).

The Elaboration of Foods into Plant Structures. — In the early stages of germination the radicle and plumule elongate by the elon- gation of the cells already present, but soon, however, in certain regions, mainly at or near the tip of the radicle and plumule, there begins cell division followed by elongation, growth, and forma- tion of tissues - — the processes upon which the continued develop- ment of the young plant depends. Throughout these processes foods are elaborated: (1) into materials to thicken the cell walls as they become thinner in stretching; (2) into protoplasm which must increase as cells grow and divide; (3) into woody and other elements for strength and conduction; (4) into fatty and waxy substances and cell thickenings for protection; and (5) into the various materials which are peculiar to food-making, reproduc- tive, absorbing, secreting, and other structures which plants form

96 GERMINATION OF SEEDS: SEEDLINGS

during their development. But the transformation of foods into the various structural elements of the plant involves chemical reactions which take place only when there is energy supplied. This brings us to another process called respiration by which the energy required for the chemical changes involved in changing foods into cell walls, protoplasm, and other structures is secured.

Respiration in plants, just as in animals, is an oxidation process in which some food or other elements are burned, as we commonly say, with the result that oxygen is required and energy, carbon dioxide, and water vapor are produced. Respiration occurs only within the cell in connection with which it will be more fully dis- cussed. But since there is no place where respiration is more in evidence than in germination where the cells are extremely ac- tive, some of its features should be noted in connection with that process. Furthermore, much about germination can not be un- derstood until something is known about respiration.

Cells, like an electric motor, steam engine, etc., can not do work unless they have energy. Some cells, like the green cells of leaves, are able to utilize the sun's energy for some kinds of work; but when cells are not specially provided with pigments for utilizing the sunlight, they have to depend entirely upon the energy which they produce within themselves. In the sugar and other foods of the seed there is much latent energy which can be released as active energy by oxidizing these substances, which are thereby broken into simpler compounds of which carbon diox- ide and water are the simplest and most noticeable ones. It is this oxidizing of substances, so that their stored energy is re- leased, that constitutes respiration, which necessarily must be accompanied by a consumption of oxygen and the production of simpler compounds. It is now clear why seeds do not ger- minate well when oxygen is excluded as the experiment in Figure 91 demonstrates. Although most of the energy released is used in carrying on the work of the cell, some, however, escapes as heat, which, like the liberation of carbon dioxide and water vapor, indicates that respiration is going on.

Respiration in seeds is easily demonstrated by germinating seeds in a closed jar, in which the production of heat and carbon dioxide with the accompanying loss of oxygen, and the accumu- lation of moisture can be demonstrated. By germinating seeds, such as Peas or Beans, in a closed vessel in which a thermometer

ELABORATION OF FOODS INTO PLANT STRUCTURES 97

is inserted, the temperature of the enclosed air may be raised 10° C. and sometimes 20° C. by the heat of res- piration; and the oxygen of the enclosed air will usually be so nearly used up that the flame of a burning match or splinter is extinguished when inserted into the jar. (Fig. 92.) To demonstrate the ac- cumulation of carbon dioxide, one may pour lime water into the jar where the seeds are germinating, in which case the calcium hydroxide of the lime water unites with the carbon dioxide of the enclosed air, forming calcium carbonate which is insoluble and when abundant gives the solution a milky appearance. Since the amount of carbon dioxide in ordinary air is not sufficient to give a perceptible precipitate, the milky ap- pearance, therefore, indicates that much carbon dioxide has been added to the enclosed air. Again, the carbon dioxide liberated in germination can be quite accurately measured by drawing the air from over germinat ng seeds through a solution of potassium hydroxide, where the carbon dioxide is caught and its weight calculated from the increased weight of the solution. However, this involves careful weighing as well as see- ing to it that the carbon dioxide already present in the air is removed before the air enters the germinator, and that the increased weight of the potassium hy- droxide is not partly due to added mois- ture. This method discloses that many cubic centimeters of carbon dioxide may be liberated by a small quantity of ger- minating seeds, as shown by the experi- ment in which 3 Beans with a dry weight of only 1 gram produced 9^ cubic centimeters of carbon dioxide

A B

FIG. 92. — A simple ex- periment to demonstrate that heat is produced by germinating seeds. The bottle A contains germi- nating seeds, while the bottle B contains only moist cotton. The higher temperature, commonly shown by the thermometer in bottle A, demonstrates that germination is ac- companied by the pro- duction of heat. If the bottles are protected against the loss of heat, or if bottles like " Thermos" bottles, which have double walls with air-space be- tween, are used, the re- sults are much better.

98 GERMINATION OF SEEDS: SEEDLINGS

during a germinative period of only 48 hours. That moisture is liberated during germination is obvious, for the air in a closed germinator often becomes so saturated that moisture precipitates on the walls of the germinator.

When green seeds, green hay, or any plant portions in which the cells are quite active are massed together, so that the heat and moisture are retained, they often become very warm and moist due partly to their own respiration and partly to that of the micro- organisms present. The so-called " sweating" of grains in the stack or bin and the heating in the bin when the grain becomes damp due to leaks are phenomena connected with respiration.

Summary. — In germination of seeds the following things take place: (1) the absorption of water which softens the seed cover- ings and acts as a dissolving and transporting medium of foods; (2) the secretion of enzymes which digest the foods and assist in other processes; (3) the transference of foods by diffusion and osmosis; (4) respiration which supplies energy for the elabora- tion of foods into plant structures and is accompanied by the ab- sorption of oxygen and the production of carbon dioxide, water vapor, and some heat; and (5) the growth of the radicle and plumule, resulting in the breaking of the seed coverings and the establishment of the young plant in the soil and sunlight.

Testing the Germinative Capacity of Seeds

The loss in crop and labor when poor seed is used may be so serious that no one can afford to plant seeds with a doubtful ger- minative capacity. It is not enough for seeds to germinate, but they should have vigorous embryos, so that they will germinate quickly and thus rapidly pass through the delicate stage in which the young plant is likely to be destroyed by insects, Fungi, bad weather, and unfavorable soil conditions.

In testing the germinative capacity, as in determining the im- purities of a quantity of seeds, decision is based upon the results obtained with a comparatively small number of the seeds as a sample. In case of small seeds, such as Oats, Wheat, Barley, and Clover, Alfalfa, and Grass seeds, tests are ordinarily made with two lots consisting of 200 seeds each and free from impurities. In Corn it is customary to use 6 kernels, 2 from near the tip, 2 from the butt, and 2 from the middle of the ear, with the kernels of each pair selected from rows as far apart as possible.

TESTING THE GERMINATIVE CAPACITY OF SEEDS 99

There are a number of germinators on the market, but, if one is not available, a box of moist soil or sand, or moist rags which are rolled up with the seeds within are good germinators when properly handled. (Fig. 93.) A very good germinator is made with two dinner plates and blotting paper as shown in Figure 94-

During the test a temperature suitable for the germination of the kind of seeds involved must be maintained. Some prefer to keep the temperature near that of the soil, so as to more nearly

FIG. 93. — Doll rag testers, consisting of moist rags properly labeled and rolled up with the seeds within. After H. D. Hughes.

imitate the soil conditions under which most seeds do not germi- nate so well as they do in germinators. The germinator should be opened each day to note the germinated seeds and to allow the entrance of fresh air, if ventilation is not otherwise provided. At the end of the germinative period, the results are usually ex- pressed in percentages found by dividing the number of germinated seeds by the number in the lot and multiplying by 100. Thus if

190 of a lot of 200 germinated,

19° * 10°

= 95 per cent. The

percentage of germination will vary for different lots and the

100

GERMINATION OF SEEDS: SEEDLINGS

greater the number of lots tested, the more the results will be checked and, accordingly, the safer will be the conclusions.

In estimating the germinative capacity of seeds, the time allowed for germination must be considered; for seeds having weak embryos and, therefore, unfit for planting may give a high percentage of germination if allowed enough time. It is, there- fore, necessary to fix a time limit, and in doing so the ger- minative speed characteristic of the type of seeds involved and the temperature of the germinator must be consid- ered; for some seeds naturally germinate more slowly than others, and the effects of low and high temperatures on ger- mination are already known to the student (page 90). Furthermore, kinds of seeds differ so much in germinative capacity that a percentage of germination considered good for one kind of seeds would be con- sidered poor for another. Thus 70 per cent germination is good for Parsnip seeds but very poor for Wheat or Corn. In the following table1 the number of days in which the seeds should germinate enough to show their germinative capacity, and the percentages of germination considered good for first-class fresh seeds, one year with another, are given.

FIG. 94. — Simple germinator. A, closed. B, open. After F. H. Hillman.

Seed.	Germination period, days.	Good germination, per cent.
Red Clover	6	90
Alsike Clover ... ...	6	90
White Clover Alfalfa	6 6	90 90
Timothy	6	96
Bluegrass (Kentucky)	28	80
Millet	5	95
Wheat	3	95
Oats ....	3	93
Barley .	3	95
Flax	3	95
Corn	5	92

1 Testing Farm Seeds in the Home and in the Rural Schools. Farmers' Bulletin £28, U. S. Dept. of Agriculture.

SEEDLINGS

101

Seedlings

After the radicle and plumule have escaped from the seed cov- erings, the young plant passes into the seedling stage, which lasts until the young plant becomes entirely self-supporting, that is,

FIG. 95. — Early stages in the development of the Corn seedling. A, section through kernel, showing cotyledon (c), radicle (r), and plumule (p). B, after germination with radicle or primary root (r) and plumule (p) much elongated. C, radicle (r) and plumule (p) much further developed; s, sec- ondary roots; I, leaves; t, coleoptile.

until it no longer receives any of its food supply from the seed. From the seedling stage the plant passes into the adult stage, ex- cept in trees where a sapling stage occurs. However, the division

102

GERMINATION OF SEEDS: SEEDLINGS

of a plant's life-cycle into successive stages is somewhat artificial, for the stages so overlap that they can not be separated. In this presentation we are chiefly concerned with the seedling stage —

the stage in which plants present differences that sometimes must be reck- oned with in choosing proper methods of plant- ing and cultivating, and that often explain the peculiar features of the plant in the adult stage. Among our cultivated plants there are four rather distinct types of seedlings as those of the Grasses, Onion, Beans, and Peas illustrate.

Seedlings of the Grass Type. — The seedlings of all Grasses are so similar in type that their essential features may be learned by studying the seedling stage of Corn. From Fig- ure 95, showing the de- velopment of the Corn seedling, it is seen that the radicle develops directly

FIG. 96. — A later stage of the Corn seed- ling, g, ground line; p, plumule; a, first node with permanent root system; 6, portion of stem between the first node and kernel; k, kernel; r, radicle or primary root; s, sec- ondary roots of the primary root system; d, permanent root system; c, coleoptile. About half natural size.

downward, forming the first root called primary root from which secondary roots arise as branches. However, not all second- ary roots arise at this time

from the radicle, for some often grow out from the stem just above or below the cotyledon. The plumule, although developing more slowly at first than the radicle, soon breaks through its sheath-like covering (coleoptile) and rapidly elevates its leaves to the light. As the plumule is

SEEDLINGS OF THE GRASS TYPE 103

unfolding its first leaves to the light, a zone, called a node, is formed at its base about 2 inches under the surface of the soil, and from this node and others soon forming above it, there arise roots of a much larger and stronger type than those formed from the radicle and from the stem in the region of the cotyledon. These secondary roots, which are outgrowths of the plumule since they arise from its nodes, constitute the permanent root system, which as the name suggests remains active as an anchoring and

FIG. 97. — Diagram showing the effect of planting Corn at different depths. <7, ground line; p, permanent root system, which always develops at about the same distance under the surface; a, temporary region of the stem, which is much longer in deep planting; k, kernel; t, temporary root system. Modified from "Elementary Principles of Agriculture" by Ferguson and Lewis.

absorptive system as long as the plant lives. After the permanent roots are established (about 10 days after planting) the first roots, which are known as the temporary roots since they serve the plant only till the permanent roots are established, develop no further and remain as vestigial structures until they finally disappear.

Also included among the temporary structures is the portion of stem between the first node and kernel. (Fig. 96.) During the early stage of germination, this stem portion performs two

104

GERMINATION OF SEEDS: SEEDLINGS

important functions: (1) by its elongation the plumule is assisted in reaching above the soil; and (2) through it the endosperm and substances absorbed by the temporary roots reach the plu-

FIG. 98. — Seedling of Wheat after the permanent root system is estab- lished, g, ground line; p, permanent root system; a, temporary stem por- tion; k, grain; t, temporary root system. About half natural size.

mule. But after the permanent roots are well established, there is no longer any need for this stem region, which now being with- out a function makes no further development. Nevertheless

SEEDLINGS OF THE GRASS TYPE

105

in connection with it, there is a principle which is reckoned with in growing certain plants of the Grass type. According to the depth of planting this temporary stem region is long or short. (Fig. 97.) This is due to the fact that the first node and, consequently, the first of the permanent roots are always estab- lished about the same distance under the surface of the soil, re- gardless of the depth at which the seed was planted. Therefore,

B

FIG. 99. — Stages in the development of the Onion seedling. A, section through an Onion seed showing endosperm (en) and embryo (e) with the hypocotyl (h) and cotyledon (c) indicated. B, seed germinating; g, ground line; s, seed; c, cotyledon; h, hypocotyl; r, radicle. C, seedling more de- veloped; c, cotyledon which is being pulled out of the seed; h, hypocotyl; r, radicle; /, first leaf. D, a later stage of the seedling with cotyledon free from the seed and permanent root system (p) developing.

a deep permanent root system, which is often desirable in order that the plant may withstand drought, is not secured by deep planting — a fact which has been well demonstrated in case of Corn and the small grains. Moreover, if the seed is planted too deeply, its food and energy may be exhausted before the plumule reaches the light, in which case the seedling is unable to continue its development.

However, after the permanent roots are established they may

106

GERMINATION OF SEEDS: SEEDLINGS

be put deeper in the soil by adding dirt around the plant. In semi-arid regions where a deep permanent root system is desired, the ground is often listed, that is, plowed into deep furrows, and the Corn planted in the bottom of the furrows. Then as the

furrows are gradually filled in cultivation, the permanent roots are buried deeper in the soil, where there is a chance for moisture during drought. In this same connection, one can see some advan- tage in drilling small grains in that the roots of the plants will be buried deeper as the dirt from the ridges is carried into the drill furrows during rains and thaws.

In the small grains, such as Wheat, Oats, Barley, etc., although the temporary system is just as prominent as in Corn, there is, how- ever, a difference of minor importance to be noted in the number of primary roots, which is one in Corn but two in

FIG. 100. — Stages in the development of a Common Bean seedling. A, the cotyledons (c) being pulled out of the ground by the hy- pocotyl (h). t, testa; r, radicle; a, root hairs; g, ground line. B, the hypocotyl has straight- ened, and the cotyledons have shed the testa and spread apart, thus giving freedom to the plumule (p). C, stage with plumule develop-

ing stem and leaves (I), root system much en- larged by secondary roots (s), and cotyledons (c) shrinking through loss of stored food.

or more in the small grains. (Fig. 98.)

The presence of the temporary system, although occurring in other plants, is a not- able feature of the Grass seedlings. Another feature to be noted is that the cotyledon remains where the seed was placed in planting, that is, it is not pushed up out of the soil by an elong- ating hypocotyl.

SEEDLINGS OF THE COMMON BEAN TYPE 107

Onion Seedling. — The seedling of the Onion represents another type of monocotyledonous seedlings. In this type the hypocotyl elongates and pushes the cotyledon above ground. (Fig. 99.) As in the Grass seedlings, the primary root system is temporary — a feature quite common in Monocotyledons, al- though in some it lives much longer than in others.

Seedlings of the Common Bean Type. — The seedling of the Common Bean is representative of those dicotyledonous seedlings in which the cotyledons through the elongation of the hypocotyl are carried above ground, sometimes several inches or even a foot in some Beans. Squashes, Cucumbers, Pumpkins, Melons, Radishes, Turnips, Castor Bean, Maples, Ashes, Clover, Alfalfa, etc., besides many of the Beans have this type of seedling. In seedlings of this type the first root system is usually the permanent one and soon firmly anchors the hypocotyl which then by an arch- ing movement pulls the cotyle- dons out of the ground in such a way that they offer the least re- sistance in passing through the FIG. 101. — Squash seed germi- soil and afford the most protec- nating, showing the peg by which tion for the delicate plumule. the seed coat is held while the (Fig. WO.) In some cases, as in ^yledons are pulled out of the . . seed coat by the arch of the hy-

the Melons and Pumpkins, the p0cotyl. Somewhat reduced, hypocotyl also assists in casting

off the seed coat, in which case the arch of the hypocotyl pulls the cotyledons out of the seed coat while the latter structure is held in place by a peg-like structure of the hypocotyl. (Fig. 101.) In most cases, however, the seed coat is torn and gradually pushed off by the growth of the seedling. Since the first root system is usually the permanent one, its depth is closely related to the depth of planting.

The plumule remains small and enclosed between the coty- ledons until pulled out of the soil. Then by a straightening of the hypocotyl arch and the spreading of the cotyledons, it is fully exposed to the light, where it develops all of the plant above the

108

GERMINATION OF SEEDS: SEEDLINGS

cotyledons. Thus most of the stem and all of the leaves, flowers, and fruit of the adult stage are produced by the plumule.

The cotyledons, which are commonly fleshy in these seedlings, enlarge after reaching the light and their color changes to green, which with the presence of stomata indicates that they function to some extent like ordinary leaves in the manufacture

of foods. However, it is only a short time till most of them, espe- cially the fleshy ones, begin to show shrink- age which continues as the food is used for growth, until much shriveled and dried they fall from .the plant. In some cases, as in the Buckwheat and Castor Bean where the seeds are albumin- ous, the thin cotyledons are more leaf-like and function like ordina^ leaves for a consider- able time, although in arrangement, shape, or size they are never just like ordinary leaves and never so long-lived. (Fig. 102.} Where the cotyledons are large, much force is required to pull them through

the soil, and, consequently, when the ground is hard or covered with a crust, seedlings of this type often fail to develop.

As to how the development of both radicle and plumule pro- ceeds until the adult stage is reached, that depends much upon the kind of plant. In most cases the radicle forms a central root which, although prominent at first, may be much obscured in the adult stage by large secondary roots developing from the base of

FIG. 102. — Seedling of Castor Bean, in which the cotyledons persist and function like leaves for some time.

SEEDLINGS OF THE PEA TYPE

109

the stem. In some plants, as in Red Clover and Alfalfa, the radicle forms a prominent tap-root which enables the plant to penetrate deeply into the soil in its adult stage.

In the Morning Glory, where the stem called the vine may be many feet in length, there is extreme elongation of the plumule. On the other hand, as in some Clovers and Alfalfa, the plumule and hypocotyl form a short thick stem, called the crown, which is barely

FIG. 103. — Development of a Red Clover seedling. A, cotyledons being pulled out of the ground by the hypocotyl (h); r, radicle; a, root hairs; t, testa; c, cotyledons; g, ground line. B, a more advanced stage, showing some development of the plumule (p); 6, first real leaf; d, second real leaf. C, a later stage, showing that the plumule has formed more leaves (e) but has elongated very little.

above the surface of the ground, and from which the branches arise that bear the leaves, flowers, and fruit. (Fig. 103.)

Seedlings of the Pea Type. — The seedlings of the Pea and Scarlet Runner Bean represent those dicotyledonous seedlings in which the hypocotyl remains short. Thus the cotyledons remain underground and the plumule is pushed to the surface by the elongation of the stem of the epicotyl just as occurs in the Grass seedlings. But in these seedlings, in contrast to those of the

110

GERMINATION OF SEEDS: SEEDLINGS

Grass type, both the stem of the epicotyl and the primary root system are usually permanent. In many seedlings of this type, the cotyledons are probably so much distorted in connection with food storage, that they could not function as leaves if raised to the light. Again, it is claimed that these seedlings can come up through harder ground by not having to raise their cotyledons. (Fig. 104.)

FIG. 104. — Seedlings of the Pea, showing how the seedling develops and the effect of different depths of planting, p, plumule; a, stem portion of epicotyl; g, ground line; r, radicle. The seedling at the right is so deep in the soil that it is unable to push the plumule out of the ground.

Size of Seedlings. — There is no feature in which seedlings vary more than in size. This might be illustrated by placing the seedling of Timothy or Clover by the side of a Coconut seedling. In general, the size of the seedling corresponds to the size of the seed. The size of seedlings is reckoned with in our methods of planting different seeds. Thus seeds, like Corn and Beans, are planted several inches deep in the soil, while seeds, like those of Lettuce, Clover, and Timothy, are sown on the surface, and cov- ered only lightly if at all. In small seedlings there is not enough food to enable the plant to reach through thick layers of soil. Tests have shown that not many Clover seedlings get through the soil when the seeds are planted even 2 inches in depth.

SUMMARY OF SEEDLINGS 111

Summary of Seedlings. — Seedlings of Flowering Plants are either monocotyledonous or dicotyledonous on the basis of the number of cotyledons. Among the Monocotyledons the tem- porary root system is a prominent feature, and the cotyledon may remain in the ground as in the Grasses or be raised to the light as in the Onion. In Dicotyledons the first root system is usually the permanent one and may consist mainly of a tap-root or of many roots nearly equal in size. In many Dicotyledons the coty- ledons are raised to the light where they function to some extent like ordinary leaves. The fleshy ones, however, lose their stored food in a short time and fall from the plant. In some cases, as the seedlings of Buckwheat, Morning Glory, and Cotton illustrate, the cotyledons become more leaf-like and persist longer, although they are always easily distinguished from true leaves. In some Dicotyledons the cotyledons remain in the soil and the plumule is raised to the light by the elongation of the stem of the epicotyl.

CHAPTER VII

CELLS AND TISSUES

Structure and Function of Cells

Position of the Cell in Plant Life. — Before proceeding to the study of the